Why do universities check for plagiarism for their internal assignments? The goal of these assignments should be to study and do research from the internet about topics and enhance one's knowledge. Not everybody comes to the university with an intention to do research.
Should the university not reward the student who works hard and did thorough research on the topic instead of deducting grades for 5-10% plagiarism?
Your assumption is that a student who did plagiarism worked hard. How hard is it really to press cntrl-c and cntrl-v, compared to students who wrote their own text? Based on your own arguments, who should get the better grade?
Moreover, a grade is not a reward for hard work. If you work hard but do it wrong, you should fail the test. So if the test is to produce an original text, and you plagiarize, then you should fail the test, regardless of how much time and effort you put into it.
The point of a university education is that you move beyond just being able to repeat what others have done. You are supposed to learn to build your own arguments, to be your own person. That is important in research, but also outside research. Do you think employers really hire university graduate because of the knowledge they learned? That knowledge is often outdated so quickly.
This is why plagiarism is such a big deal in universities. It is not because its link to research, but it violates the central goal of a university education. You are lucky to just get a lower grade. In my university I would have to report you, and that would probably have led to you failing the entire course, and if you are a repeat offender you could have been kicked out of university...
Should the university not reward the student who works hard and did thorough research on the topic [...]?
Of course. But copy-pasting text from the Internet is not "working hard and doing thorough research". You can copy-paste something from the Internet without even reading it, let alone understanding/learning it.
On the contrary, if you work hard and do thorough research, you will typically read several sources, compare them, and write your conclusions on them. If you write things in your own words, it means that you understood them, and so you deserve a good grade.
In addition, passing other people's work as your own (which is the definition of plagiarism) is not ethical, both inside and outside academia. Of course it is more strictly monitored in research, but it does not mean that it cannot get you in trouble if you do it somewhere else.
So university is not exclusively educating you for research: it is teaching you both technical content and conduct guidelines that will be useful to you in your professional life.
Other answers and comments have pointed out the flaws in the assumption that our hypothetical plagiarist has in fact done significant work in copy-pasting their assignment. But this is only part of the story. It's not worth very much if you can find facts but can't understand them. By requiring original work, we (try to) ensure that the student actually learns something from the facts they find.
In fact, I'd argue (and I don't think I'd be alone in this) that the task of putting content into your own words is how one gains an understanding of that content. So this isn't just work the student is doing to convince the teacher of their understanding, but rather an important part of building that understanding itself.
There are at least two researchers who have published extensive literature on this subject which contains more explanations than could be put into a concise Stack Exchange answer, but I refer you to the publications Robert Clarke of Birmingham City University, and Debora Weber-Wulff of HTW Berlin.
You have overlooked one particular aspect plagiarism that universities are concerned about which is known as "Contract Plagiarism". Contract Plagiarism is where a student pays an another party to do their work for them. This is where the student has done no work whatsoever. The student would not have done internet searches on the topic and copied fragments, nor had discussions with class mates and shared some material.
Internet tools are an important weapon in the war on this form of cheating, which is basically degree for money. The activity debases and devalues the degrees which we all hold and worked so hard for. If it is not tackled head on every qualification would come under question.
(Thanks for the reference, Brian Tompsett!)
In order to demonstrate the research you did, it is important to carefully reference your sources. Not only do you note down where you found the information, but you mark exactly (with quotation marks) when you are using the exact words of the source. If you are paraphrasing, you have to make sure that you make the beginning and the end of the bit you are summarizing clear to your reader.
Why bother, you may ask? Well, you want to build a convincing argument! I had a student the other day try to convince me in her thesis (without reference) that Internet Explorer was the most-used browser. I asked for her source in the oral exam, she replied that "everyone in the office I work in uses it". That's a pretty poor excuse for research! And it will lead you to producing bad products.
And even when you are programming, you can insert references to the code you are basing your work on (or the Stack Exchange post) by putting the URL and additional information into your code in comments. Your future self will thank you for that.
Plagiarism software is often misused as a diagnostic tool. It can find some plagiarism, but it both misses a lot and marks correctly referenced material as plagiarism. So it is only a tool to be used by instructors, not as a judge of your work.
As others have said, you come to university to learn to do research (much needed in industry, who wants to re-invent the wheel constantly?), to think complex matters through, and to communicate your thoughts and arguments to others orally and in writing (and also in foreign languages and programming languages). So yes, we check for plagiarism from the start. And if you are caught, there will be severe sanctions. Science is built on trust. If you violate that trust and pretend to do work that you didn't, then you deserve the sanction meted out.
I did my undergrad and graduate degrees in the 90s, and it's worthwhile comparing to the situation then. The internet did exist (email, usenet, and -- gasp -- browsers!) but indexing and search were poor.
In more technical subjects, we got assignments we were supposed to figure out on our own, to test and expand our knowledge of the material. Yes, sometimes someone found a worked solution to the question, and saved some time copying it versus figuring it out on their own. But hunting through sources in the library was educational on its own, and by hand-copying out the solution you couldn't but help internalize it, learn from it, and generally write it in your own words a bit. So educational objective realized, if perhaps suboptimally.
In less technical subjects, when we wrote papers the expectation was to research, and then articulate in our own words. Once again, absent the shortcut of Google search-Copy-Paste, mindless copying was less of an issue. Did students sometimes plagiarize? Of course, but the extent of it, and the impact of it on their understanding, was much less.
"Cheating" did happen, of course, on assignments, and sometimes even by "buying" papers. But professors who found that an issue modified their assignments over time so it became less attractive and easier to detect.
While the situation of course varies from case to case, today's frequent student plagiarism comes from a pernicious assumption that Search is an alternative to actually thinking about a problem, rather than actually engaging with it. By and large, correctly detected plagiarism (and there can be errors!) does not penalize students who "thoroughly researched" the topic; it catches those who searched-copied-pasted in lieu of researching -- and often haven't yet realized that research goes beyond search!
Finally, to your point about "not everyone wants to do research". That is true, and I've since also managed teams doing applied R&D in industry so I'll bring that perspective as well. When a team member says, "I've found this interesting approach to our problem on the internet, and here is why it's relevant, here is how it's different, and therefore here's what we should think about before using or adapting it", that is super valuable. However, "Here, I've solved our problem! [And it looks like I did something impressive, but I actually just found something on the internet and hit Copy-Paste]" is not, primarily since it's much likelier that person cut corners assessing if it applied well, thinking through complexities that might be missed -- as well as might have swept under the carpet IP (intellectual property) issues that might be significant.
"Plagiarism" on school assignments is more like the latter than the former.
In addition to the other answers, this:
You're supposed to use the undergrad assignments to learn how to properly cite sources. It's not trivial - different fields and different journals have different citation styles. Also, different sources need to be cited in different ways.
You need practice to learn to do this correctly. Quite aside from forcing you to be basically honest, the anti-plagiarism rules also force you to put in this practice.
Plagiarism is very bad. In the outside world it can you get fired and cost your company millions. Newspapers which publish plagiarized stories take years to live that down, even if it was entirely 1 reporter's fault. Software with "borrowed" code gets sued to death. Likewise engineering projects. Joe Biden was mercilessly mocked for plagiarizing a few sentences in a speech.
A school doesn't want its grads to keep getting fired and blacklisted. At the very least, it's bad for the school's reputation.
In a practical sense, it's each instructor's decision. No one tells us specifically what to spend more or less time on. but we generally put writing up blatant plagiarism pretty high on the list.
