Why do people graffit vulgar things?

Lots of silly boys think that is funny, since forever.

Shock value possibly. Like any youth craze, Beatles, punks it's intended to try to upend the establishment. Even Beethoven was considered a rebel of his day.... and then you become the establishment with your job and mortgage and the new thing comes along. Unlike tagging of course which is what mammals do to mark their territory which is just like peeing up a lamp post but with a spray can. 

The devil finds works for idle hands.

It's not that tragic, actually. Somebody did the same to my uncle's house facade, that was nasty. That "is" nasty, actually, it's still there...

They're all nose pickers and booger eaters who think they haven't been caught.