Yeah I know you can race and whatever, but the main reason is to kill other players.
I kill other players after finishing missions with them. I beat two guys at golf then after gunned them both down with my combat gun and drove off. I use my tank and blow people up, I couldn't care less if it costs me money and taking out their PV.
I reverae back in my Entity and shoot people. Let them blow me up and pay up. I drive my PV into other tanks.
Its all fun, and if someone kills me I don't take it personally. I even kill my own crew members.
You control a criminal thug. All this loyalty and playing the game fair is stupid. I have a shoot on sight policy and it's not going to change.