If someone answers a question, and another user edits it by adding on more information, should the edits be approved? On the one hand, the post is improved by the edit, but on the other hand, shouldn't these additions be suggested in the comments, or part of another answer altogether? The poster clearly did not include the info that the editor stuck in; I don't mean an oversight.
If the edit makes an improvement, by a small addition, a spelling fix etc, then we don't tend to worry about it at all.
If it is a major change then we'd tend to reject it - something majorly different should be your own answer.
The challenge is that middle ground - and here there are polite workarounds such as suggesting in a comment that something should be added, and then either the OP can make the edit, or someone else can. And once that is done, the comment can be removed. This is one of the specific reasons for comments.
An alternative, which is a bit less polite but still often acceptable, is to make the edit and leave a comment in the edit reason box to explain why. If the OP dislikes the edit, or if others agree it is a bad edit, it can be rejected or even rolled back.
The end goal is to have a good answer, so we encourage editing to do this. We just encourage politeness along the way.
Edits should serve to clarify and improve posts, while preserving the author's original intent.
There are two kinds of edits that I look particularly hard at. Formatting edits often seem capricious; if these clearly improve the readability of a post they are fine, but if instead the improvement is questionable, e.g. placing some words in bold because the editor thinks they should be emphasized (i.e., opinionated edits), I tend to reject these as "No improvement whatsoever."
Edits that add content to an answer may improve the answer, but do such edits deviate from the original intent of the poster? Maybe the original author wanted to avoid muddying the waters with less relevant details, or maybe the original author would disagree with the position taken by the new content. If an editor has new content to add, generally they should write their own answer rather than embellish an existing one; it is perfectly OK to write an answer to an old question with good answers if the new answer adds something.
For me, the question that must be asked of an edit is, does it clearly improve the post in a way that is not primarily superficial and opinionated, and in a way that does not deviate from the original author's intent.
In adding extra information, it's changing what the OA has stated. That may have been omitted for several reasons - keeping an answer precise, simple, or just not thought of. Any extra info needs to be a separate answer, from that editor. Or at least in a comment under the answer. Editing of spellings, syntax, etc. are acceptable - nothing wrong with good grammar, as long as it doesn't alter any meaning from the OA - unless it's a glaring incorrect statement, which again benefits initially from mentioning in a comment - politely, of course!