In many of my undergraduate classes, professors required up to date editions of textbooks. This was across a number of subjects from Computer Science to Accounting and across introductory and more intermediate levels.
What is motivating new editions of textbooks? Are professors who published contractually obligated to publish new editions? Are they incorporating student feedback?
Publishers want new editions so that they can make money selling copies of the new edition and reduce the market for used copies. The new edition might be significantly updated, but in many cases the updates are small. For textbooks in lower division general education courses, new editions come out as often as every three years.
It's quite common for textbook publishing contracts to include clauses that give the authors right of first refusal to produce an updated edition but allow the publisher to add a new coauthor and produce a new edition if the authors are unwilling to do so.
There're many possible reasons for writing a new edition:
Authors are not usually obliged contractually to produce a new edition. At most, they might be contractually obliged to publish new editions with the same publisher.
I am an author currently updating a textbook. It's been out for several years. Many of the exercises are based on "current news" - what was current then is not current now. While updating the exercises I've found more places than I had anticipated where I see ways to say things better.
I am arranging the new version so that a second hand copy of the old version will still work. Exercises I've removed will be available on the web with their original numbers, new exercise numbers start where the old ones left off.
To answer the more general question: I think that the point of many new editions is new revenue.
When sales start to lag on a popular title, publishers want a refresh. They want to try to boost sales back to where they were. Adding a chapter on new material is relatively easy. Adding or changing exercises makes it harder to use older editions for adopters.
Note that authors normally give up copyright to the material so publishers are free to leave authors behind in the preparation of a new edition, but are unlikely to do so even in the absence of a contract. This is because adopters often (usually?) associate the book with its author as much or more than with its title. So including the author has value. But, as Brian Borchers says, there is usually language in the contract about this.
I think it is very unlikely that student feedback is used in the preparation of a new edition, other than from students of the authors. But book representatives (acquisition editors) usually attend professional conferences and ask for feedback on books from attendees. They will also sometimes poll adopters of the book to get feedback and this can be given to the author to aid in the preparation of the next edition. Some of that feedback is contradictory, however, and some is contrary to the ideas of the authors.
In fact, some acquisition editors will ask for feedback on the (popular) books of competitors to learn why those books were chosen instead of their own.
Have you ever been involved in someone teaching a lecture from a "script"? I don't know if there is a specific English term for this, I mean that the teacher/professor collates the material beforehand without publishing it in a book form, and makes it available to students.
If you see this being done year afer year, you will see what kind of changes are made.
The importance/length of sections is changed relative to each other, due to changing emphasis, the need to make space for new material, and the time needed by the professor and/or students to get through a section
practical problems are changed to be more understandable, new problems are added, etc.
material that was difficult to bring across is rewritten to be presented in a new way
small new discoveries are mentioned, such as using the results of nifty new studies as examples that emphasize a point
corrections are made, since there are usually errors at the beginning
I would say that textbooks go through similar changes between editions. This is entirely normal - a large and complicated artefact like a textbook is best created in an iterative manner, not unlike a software programm.
Major discoveries in the field are much less likely to trigger a new edition. First, the future impact of many discoveries is not recognizable when they are made, and they linger in some small journal before the discipline notices them and makes something out of them. Second, even when something is recognizably new and different, and excites scientists, it is still not "fleshed out" enough to be taught to students, since it doesn't yet have its own ecosystem supporting literature, successful application in large projects, whatever. Third, the kind of professor who gets to publish a textbook is usually old, experienced - and set in his ways. If he dedicated his life to building superconductors out of metal alloys, and some young upshot shows that graphene can be used in a superconductor, the professor will wait for a few years whether that new technology (which is in direct competition to his own research) will establish itself, before starting to give it space in his textbook.
I think there are a few exceptions to that "discoveries percolate slowly into textbooks" tendency, for example I heard somewhere that CRISPR/CAS entered general genetics textbooks rather quickly. But it is much more typical, especially in undergraduate level textbooks, that changes between editions are incremental improvements of existing material.
There are also some fields where the changes are very impactful. This happens in fields which study human-created rule systems, typically law, but also accounting. In a law textbook, a subset of laws changes every year, and their interpretation by courts also changes with new case decisions. The new editions of textbooks have to reflect these changes.
A reason for professors to want the newest edition (beside monetary ones) is simply that it makes it easier to teach. With a class where the students use multiple editions at once, there will be difference in the text, but especially also in the problems. Making sure that everybody reads the same text, or is solving the same problem when homework is given, is a huge headache if students use different editions.
They can update a book for several reasons:
1) new material,
2) updating material (addition or removal), chapters or sections based on feedback - changing the order of sections
3) more examples with solutions and/or practice problems with or without solutions
So technical books are updated as necessary but they are not done for lucrative reasons - fiction authors sell more copies and do make money... technical books don’t sell in the same numbers...
In something like English, clearly the field is not changing hugely -- incorporating technology has been done pretty well. (Although I want to update a book I used in Tech Writing because it was pre-google docs, and assumed emailing around a single document, taking turns editing.)
Mostly instructors find other approaches that work, and they want to create a text to allow other instructors to use the same one.
I often would incorporate a chapter from a textbook I had a sample from, while my main text for the class stayed the same. I didn't outright steal (Xerox or scan it in), but I'd take the organization of the information for that chapter, and make a presentation (PPT) based on that, and come up with my own examples, and find exercises in my existing text that could be adapted. This was a tech-writing class, and that other book went into more depth on layout topics and strategies. To get even a portion of that book for the class (I wrote the publisher and asked for a 2-3 chapter excerpt) would have been $30/copy, and my main book in that class is only $30ish already. If my preferred text lacked adaptable examples though, then I may have switched or added in that supplement fully.
Some teachers in my department ADORED a text that gave a lot of "models" for how to do academic writing, but I despised it. So I'm glad other people had created books that focused more on the analysis and deep-thinking parts of the writing process. So while not new editions, they were various approaches being represented. (I did stop using one ENGL 100 book when 3rd edition was 250ish pages, but by 6th, it was 550 -- each individual change made sense, but it was overall too overwhelming for a student to cope with.)
20 or so years ago while in the army, we had some rule book (The Rules of Military Discipline, printed in 1965 I think) which were erata-ed the hard way: new text written by typing machine, cut and pasted (glued) over the old text.
As another example, children today learn Mathematics at least one year earlier than I did in the eighties, so new manuals are necessary. Also, the things that MUST be studied (government mandates what) change - if not every year, then at least once every four. While dealing with things that disappear from the manual is easy (just ignore them), the changes are all ADDING things.
