When it comes to editing answers you generally have to be much more conservative with your changes. Only copyediting is really allowed there. Even changes that incorporate comments, whether posted by the answerer or others, will change the nature of the answer. Unless it's a community wiki, the author of an answer has final say in its content.
Questions can be edited much more freely. Comprehensive edits are typically to clarify/highlight the core question, cut out meaningless fluff or incorporate comments by the OP. Question submitters rarely do that kind of edit work themselves and very rarely make comprehensive edits instead of tacking on updates. In contrast with answers, in practice there is no assumption of ownership when it comes to questions. Coupled with the fact that the language level is typically vastly higher in answers than in question, the end result is that answers are rarely edited which means that the collaborative aspect of question editing is largely absent from answers.
There are only a few commonly accepted edits to answers:
- copyediting: typos, grammar, general language cleanup
- clarifying concepts or abbreviations: with short definitions in brackets or, more typically, links to definitions/Wikipedia
- adding references: less common on this site but we still occasionally get things like "I've read an article before that said X" or "this was discussed on another question"
- disclaimers for outdated advice (these edits are usually highly contested, but sometimes necessary, less so on this site)
This basically matches the description of the editing privilege. Note that while the same standards theoretically apply to questions as well, in practice you have much more freedom to drastically edit questions, provided that those edits are beneficial and don't change the core of the question (unless the latter is approved by the OP or the question as-is is unsuitable).