Sometimes, mods delete comments. In fact, sometimes mods delete entire comment threads. This inspires people to come to this meta or site-specific metas to ask "why were my comments deleted" (e.g. here, here and here and many many others.) There is even a tag that seems to be specifically for this thing, deleted-comments.
Although I would not go so far as to claim consensus on the matter, almost universally an answer is given and receives upvotes that hinges essentially on a line from the comment policy "Comments are temporary 'Post-It' notes left on a question or answer." This answer generally boils down to, I think "Comments were deleted because comments are by nature ephemeral and therefore are subject to being deleted." (e.g. here, here and here). However, in giving this as an answer to "why were these comments deleted?" it seems to suggest that not only might comments be deleted but, in fact, comments should be deleted.
However, at least one of the "should"s on the comment policy suggests that some comments should possibly be at least long-lived. Specifically: "Add relevant but minor or transient information to a post (e.g. a link to a related question, or an alert to the author that the question has been updated)." The phrase "minor or transient" and the example "a link to a related question" suggests that there may exist comments whose use is not transient, but is instead minor, and may be useful for longer than it takes for the OP to read and comprehend it.
So, is it the case that all comments should be deleted? If not, then there exist circumstances in which a comment should not be deleted... what are those circumstances? How can we tell the difference between comments that should and should not be?