We post comments to posts which lack effort, suggesting them to include their efforts(I agree with that) or informing them that the answer is trivial when they use a particular formula or method.
Alright, it's good that we do not provide answers to homework questions when the person who posts the question doesn't show their efforts and notify them in the comments.
But some users, post negative comments to low-quality posts and answer the same(contradicting their comment) when other users show no or little interest towards that post or their comment. Once they post an answer, they delete their negative comment for the sake of gaining reputation.
Users should stop doing this kind of activity. (My post may sound a bit rude, but this happens quite often in Math SE.) When users are intended in posting an answer why should they post a comment contradicting it?
This post shows that only diamond mods can see the deleted comments and even they have to explicitly look for them.