Just marking their territory. Graffiti has been around practically since the dawn of human civilization, not like Black and Mestizo gang bangers invented it or anything. There has even been Viking rune graffiti found on old architecture from Rome and even as far as Baghdad, I guess those Vikings really liked to leave their mark. You should see all the graffiti and street murals the IRA types and Loyalists have all over Belfast and Derry, and its been like that since long before this gangsta/hip hop culture ever existed.