I've seen many acoustics with it. Does it actually serve any purpose? I also saw Passenger play with a decorated one his the music video for Scare Away the Dark, where can I get one like that?
I think you mean pick guard. They are intended to reduce damage to the top of the guitar caused by aggressive picking or an anchored finger on the picking hand, and as such have a useful function for some players and not others.

Those decorated guards a typical of some Gibson models, and have been copied by Asian makers of Gibson knockoffs. A Google search for something like "Gibson style pickguards" should find some.