"Mark thread as read" button, just curious, why?

hey I'm just curious as to why someone would want to mark a thread as read when they haven't read it...

It's handy if you have two places to read forums, like you know you've read all of the post because your reply is most recent or something.

It's also handy if you don't care about certain spam threads and just want to see new posts in the forum. It's for people who want to read everything, and use the unread indicator a lot.