Personal transport - after all you cannot drive everywhere in a tractor!
Why own a car/truck? Couple of reasons - first you can park it at one of the Mission Signs to make it easier for you to quickly switch to the board to flag Missions or just check to see what is available.
Second, you can use them to rack up the miles if you are looking to unlock the driving Achievement / Trophy..
Finally, well, you know, its a car or truck! Heh.
How do you get money