Sometimes tagging is that, sure, or just some person indicating that they exist there. For some taggers, it's an addiction. I knew one that would tag at people's houses when invited to parties. I was outside smoking a cigarette with him after the owner had threw him out on his ass, asking why he did shit like that, and he said "I just feel like if I can tag someone's house, it's like I've won."

I can kinda empathize since I'll have an addiction to getting the perfect photograph during a protest or whatever and will go to extreme lengths and burn through SD cards to get it.

In my experience the majority of graffiti is artists just putting up art. Privileged folk pass down the propaganda that graffiti is dirty and gangster and so any street art is viewed as dirty, but in the end it's just a matter of taste.