Comments are intended to offer suggestions on how to improve pots (questions or answers). Technically, Comments that do not do that may be deleted at any time. Comments that have already been acted on are often deleted. Comments that clearly will never be acted on are often deleted. (Note: if a comment is copied into a post, in whole or in part, proper attribution must be provided, and this may require retaining the comment.)
Comments are often used to offer additional information or different perspectives on a post. Technically such common may be deleted at any time, and some mods routinely do so. I think this is unwise, because such comments are often helpful. As a recently elected mod over on ELL, I find myself more reluctant to delete possibly helpful comments than some mods are, but more willing to do so than I had expected to be.
Here on Law, if a comment points out a typo in a post of mine and I fix it, or a comment suggests an idea I should mention and I do so, I routinely flag the comment as "no longer needed". If I do not make a change based on a comment, I do not so flag it. But no rule requires that exact pattern, s far as I know.
Clarification: The statements above are not my personal views on how comments should be used, they are a summery of things I have been told at various times by moderators, high-rep users on sites I was active on, statements I have read in the Help Center, and much-upvoted posts on MSO. Some of them I would perhaps change if I were asked to rewrite the rules for SE. But the above is my best understanding of agreed policy for SE, of what mods and CMs actually enforce, an what TPTB want and largely expect users to follow. Some of this has been reinforced by topics I have been pointed at since (very recently) becoming a mod over on ELL. I don't yet have any sufficient experience at that to rely on it.