Mj.

They make leaving posts to inform people who aren't "in the know," as it were, about why they left and if it's permanent, etc. It's really not that big of a deal, but whatever, grr attention whores grr

because people can actually be rather close friends online, and feel the need to let said friends know what's going on with them

you could always do it on skype yeah but whatever, its not like it harms anyone

It's a community, and people like to feel apart of that community. When they leave or decide to take a hiatus, a thread may seem appropriate to them because the community has had a pretty profound impact on them.