Without the ability to go back and edit (or append to) a previous comment, replying to yourself is about the only way to give updates or to give further details, or clarify, etc.

I've gotten in the habit of upvoting comments on SN as a way of marking them as having been read by me, so when I revisit a post, I can easily tell what I haven't yet read. It has the additional affect of letting the author know that their comments are being read.