People use it for different reasons. Aside from those Koala gave, I think the original intent was a way to thank a poster for the post or something in it (comment, link, reference, mention, answer, entire post... whatever) without using another post just to say thanks. It can be tedious reading a post and then umpteen follow-ups just saying, "Thanks".

"...Or 'El Duderino' if you're not into the whole brevity thing."—The Dude

Doc