All graffiti, including tagging, is a person's expression of control over their environment. There is a reason why the suburbs aren't filled with it but the poorest places are, graffiti does not appear randomly, it appears most frequently in places where the people have the least control of their lives and this materially expresses itself in graffiti as an attempt at taking some control of anything they possibly can.

edit: I would go so far as to say it is a natural feature of struggle, in the places where the most struggle is occurring, and to dismiss it is also to dismiss the people that are in the material conditions that are most likely to benefit from becoming organised. These people are some of the very most likely to be communists if you could only reach them with theory.