Reason? You make over $150,000.00 tax free selling drugs on a corner. One day someone else is on your corner so you KILL them.
Decades ago, people got drugs from their "connection". You knew somebody, who knew somebody, and he would supply you with what you wanted.
Then police started adopting the tactic of making dealers rat out their buyers. This made it necessary for buyers to become anonymous, so sellers had to secure a location where they could be located. Thus "turf wars" over locations which had accumulated lots of traffic.