The previous post is older, let's say a day, a week or maybe a hour only. You would want to write something in the thread to inform everyone about a status change of something. You can't post adding at the last post because you could be sure that no one will realize the last post changed. No update of the thread would be propagated and the info would be missed by a lot of people. It might even be needed to help others out, like I do in cannabit thread. It would be not possible to wait until another poster asks for updates.
Something I found working for myself when I wanted everyone to notice an edit/addition to the last post I made is copying the post, adding what I have to say, and then deleting the first one.
I don't double post that way and everyone gets the info about a new reply in their list - for me (myself), that's the solution I prefer.
If you edit a post (and you are the last post in a thread), the thread will reappear in "Watchlist" and "Show new replies to your posts" even if people read the previous version of the post. However, it will not bump the thread in the section overview.
This is, of course, if I'm not mistaken, but I'm 99.9% sure.
Didn't know about that. Good to know.
Hmmm, strange. I was pretty certain that worked as I saw it written down somewhere, guess not. My bad.