The graffiti perpetrators have this idea that "tagging" endows them with some sort of territorial status, a bit lik a dog pissing on a lamppost to mark its territory.
Unfortunately the lack of artistic merit of most graffiti reflects the state of mind of the "artists" and their desire for some sort of recognition.
The most annoying is the crap that gets put on motorway bridges, some of which, as I recall from a few years ago, was pasted up on posters by some sort of religious cult.
I guess graffiti is just another annoyance we have to tolerate, a bit like fly tipping and litter dropping.