Whats the point? Because after you beat a game like say GTA:SA, It's crazy fun to just go around a blow shit up without the fear of being taken out by some two-bit cop while your in the middle of your rampage! Or perhaps you never know when you might want a tank or jet to appear out of thin air.
Or spawn a hovercraft on top of Mt. Chilead and see how far across the map you can glide/float in it. Also, jetpack spawning. JETPACK!