Come on now, you can't get good at something without practice. Just think of it as Banksy-to-be's working on perfecting their art!
Well, that may explain some of it at least... I think there's just something in human nature that draws some people to graffiti, it's been present throughout human history (if you want a chuckle, look up Roman graffiti). Doesn't make it any less obnoxious on occasion though.
I can understand the practice makes perfect scenario, but to see an blank wall and scribble any old crap just seem a bit pointless to me. They could at least try and do something worth looking at.
Oh I know, most of the "I was here" graffiti is just obnoxious. Sometimes though I see in it places that makes me wonder how in the hell they even were able to get access to, those I find at least mildly amusing.