For people that we personally meet or were about to, if we have irreconcilable differences AND it devolves into back/forth online banter, then we have blocked people. Pretty rare, but it has happened and is likely the primary reason block exists.
We have a number of people that visit our profile a lot but apparently don't want to meet, but that really doesn't bother us. Kind of flattering in a way.
As for forum folks, if you look up the definition for "internet troll" and I get the impression their intent in most/all of their posts meets that definition, I have blocked them. I avoid the Politics thead these days mostly for this reason.
I'm sure there are other reasons to block and ironically, I noticed I've recently been blocked by a certain couple that periodically deletes all of their posts. Only reason I know this is when a blocked person posts something, you'll see their profile name in a much smaller font on the forums page. Not sure why they did this but everyone can have their own reasons for blocking.