There is a reason behind each one that gets deleted: To avoid an endless comment exchange that should be taken to chat, for example, or generally because they become obsolete, like "upload a picture" "done" "thanks". Comments are meant to be used for clarification or to add relevant information.
Relevant SO meta post: How do comments work?
Comments are often used to ask for clarification on, suggest corrections to and provide meta-information about posts.
Even more relevant, about comments:
Comments are temporary "Post-It" notes left on a question or answer. You should not expect them to be around forever: Once a clarification has been made, an edit added to the post to include new information, or the issue in the comment is otherwise resolved, it is subject to deletion. In reality, many obsolete or chatty comments remain untouched due to the high volume of comments posted, but this does not mean that they can't or shouldn't be deleted in the future.
Of course there is benefit in deleting them, most are like the ones above. We get new comments all the time, to new and old questions. Old questions don't get bumped, so those comments - lots of spam too - if lucky, are automatically flagged as low quality and we get notified. We (mods) get around 20 flags per day, most are comments.
In reference to your answer to my other meta question, some comments were deleted in this one here, and as I wrote in your answer, that exchange was turning into a flame war - Citing: the first comment, your comment, started with "thanks for the laugh". Then it got progressively passive-aggressive. A mod deleted them.
I don't mind sharing the details of each deleted comment. I think very well about them before I delete them, I only do it it it will benefit the question and the site. I think all mods in SE have similar numbers on edits and deletions. I delete and edit a lot so I take responsibility.
I visit the site each day and spend a long time reviewing, editing, leaving comments and when I can, answering. If you go through my log and feel I have edited too much, I am more than willing to review those comments and adjust my deleting.
But if I am doing too much pruning, I will have to repeat myself that moderation should be about doing as little as possible. When I visit SO I go straight for the answers and look at the comments for clarification, exceptions and such. We do design, so maybe we should allow more 'noise' in comments.
I think we need another meta post for guides on when to delete comments.