What are the motivations for publishing new editions of an existing textbook, beyond new discoveries in a field?

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:

• New discoveries in the field (e.g. detection of gravitational waves)
• Removing outdated material (e.g. if an exercise question involved a lecturer using transparencies, it would make sense to switch to a lecturer using Power Point)
• Change in syllabus (e.g. new discovery means courses should cover that, and to make room, another topic is removed)
• Student feedback (as you mentioned)

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.