It would be really cool if players could use spray-paint to write on buildings and walls. At least I believe it would be really cool.
One way it could possibly be set up is that in the main menu of the game, where you can change your name, you could also set your character's "tag". The tag could be limited to 4 or 5 keyboard characters maybe, and then in game, if you have spray-paint in hand, you could press a button or select a menu option when interacting with an appropriate surface and you would draw your tag on the surface. Maybe could use some crazy looking font or just sand font but that doesn't matter much.
How it would benefit the game is that it would allow users to have a personalized mark that they could leave behind. Clans or individuals could use the graffiti as a way of marking territory much like gangs do in real life. (Not that I agree with gangs doing that in real life but I could definitely see people using graffiti in a post-apocalyptic world.)
It could be used for warning people to stay away, letting friends know where you are, claiming territory, or just for saying "Hey I've been here!" to anyone who sees it.