I've always wondered about this, growing up in a city where street art was on every wall, and there was always some new painting or "tag" on a wall that was previously clean. I always saw people painting their walls white again over pre-existing graffitis, and then those white walls ended up graffitied again.
I always wondered what the point of it all was. It looked in so bad taste to me before, but then I started growing to understand it: bubbly texts, stylised, colourful. Still, I never got what the point was except some kind of "pride" of leaving a legacy. I actually met a couple taggers. I asked them about it and they were simply proud.
Some walls are simply destroyed, in my opinion, by graffiti, but then, walking by sites with old graffiti that hasn't been cleaned for years, it does give a little nostalgia to look at old wall drawings that were already there when I was growing up.
Why do you think graffiti "artists" do what they do? What do they get out of it, and what is the different part in their perception that makes them think that this is aesthetically appealing?