I've never understood either. In fact, I think it makes it harder to respond to posts in an earlier thread.
It definitely causes the "we've discussed that 16 times before" syndrome
I poked around, and it seems that some people believe that the longer the thread, the longer it takes a page in that thread to load. That's not my experience.
But it's also not my place to challenge a long-time practice that keeps people here happy....so that's my last thought on the matter.