I've seen this many times, and have done it a few times myself. Someone might have some words they want to include in the comments which don't necessarily pertain to answering the question or maybe even are brief statements which seem chatty. Whatever the reason might be, the person who posts it deliberately includes something along the lines of
This comment will self-destruct. Then, the comment is deleted soon after, as to not pollute the comments.
I'm just wondering how appropriate this is, especially since a majority of what I've seen are primarily based around sarcasm, and some people take offense to things like that. Of course I can't really provide any examples here, because they wind up deleted, but I'm sure you know what comments I'm talking about.
One example of a comment I posted once:
[This comment will self-destruct] "My car isn't running like it should. Please fix it Mr. Mechanic!"
I said this comment with sarcasm deliberately so that the OP could understand that the question they were asking was far too vague for anyone to be able to answer.