You did very similar work on your own before, and now you happen to have stumbled upon a similar assignment. Why exactly would reusing your previous work be unethical?
If the university has such overlapping courses and lets you take them, why should the student be punished for optimising his time/resources?
That's not that easy to answer because it really depends in my opinion on what is plagiarised and why and what's the impact.
For example we submitted a paper and the publisher found a plagiat in our introduction. Turned out we had written the same sentence in one of our other papers on a very similar topic. We had around 15 papers in that field and well, the introduction is always basically the same so sooner or later you will write down the same sentence. It's just stupid to forcefully rewrite the same introduction over and over again and it has nothing to do with the actual scientific work.
And while I feel it's unethical to use previous work in university courses the even more interesting question is: why? Why would I reuse my previous work for classes at university rather than learning something new/training my skills. Why would I study, if I'm not interested in the subject.
You really should take courses at university as an opportunity to learn something, rather than an annoying obstacle on your way to your degree, which you want to avoid/manage with the lowest effort possible
The main reason behind banning self-plagiarism is the implication that any research you publish/hand-in is not only original but also new. Whether the second part is indeed implied is dependent on the context. However in a scientific paper it might be very relevant for the reader to know where and when something was first published. For a professor, it might be very relevant to know that parts of an assignment were already written, so the student didn't learn anything new from writing that part.
Given that it can be relevant, I'd suggest a "better safe than sorry" approach here. Also: if the work can stand on the merits of only the new work, why not cite? If it can't, why rely on adding previous work without citing it?
The reason - at least at our institution - is that you cannot earn points twice for the same work. Imagine an extreme case in which you take three courses, all of which require writing an essay. The idea (and that's what's in the approved curriculum) is of course that by the time you graduate, you have shown to write essays on three different topics. If you manage to "trick" the system by finding a topic that is at the intersection of all three courses so that you handed it in three times, then you obviously have done only a third of what was expected (and thus have less experience in essay writing than the diploma you get suggests).
Of course, there are less extreme cases, but the same reasonig also extends to, say, copying half an assignment. Obviously, I am not talking about trivial things (like using occasionally the same sentence) here - that's quite unavoidable.
This is specific to coursework, not journal publications. Atleast part of the credit for any assignment is for the efforts and/or time put in. In a pedagogical context, the output is not as important as the process and what the student learns by following the process.
I should also mention that optimizing time/resources is not something that overrides learning effort, atleast for a student.
