Graffiti: For what purpose?

I was on a train recently, well, many trains. Each ride had something in common, with that being going past very rundown and slum-like cities. Safely away from the train of course. But what really got me thinking was all the massive amounts of graffiti. Most people (I assume) do things for a purpose. Making a huge visual mark somewhere can be more meaningful than we think, and not just "gang bangers" making a mess out of the walls.

Still, I wonder why in such small rusty looking towns that people would leave such marks. Many unreadable. Sometime's names are on them. The big point I am making here is it's almost like a whole society that's gone, and the graffiti is like messages to those who might be around or knew of that society. I think I am looking way too into this, but I don't know, long train rides and spray-painted walls make me think.

But yeah, what's your take on this or similar?

On public property it's vandalism and people shouldn't be doing it.

Why graffiti? Reasons vary.
1. To make a statement. I'm here, see me.
2. Art to some.
3. Rebellion. You can't stop me from messing up this wall.
4. Marking territory. Gang signs and the like.