Waiting for the used lot to produce a car you really want can be tiring, so if you purchase a car that someone else might want, why not trade it for something you want? Also, you could buy a truck for the truck even, and get rid of it after the race if you don't plan on using it again.
It's not to complete the game any sooner, it's to get the cars you want. They are earning the cars themselves, by way of purchasing the cars through other gamers rather than the dealers.
I fail to see how this is a problem. Several games, even other racers, have systems like this, often even more robust.