Enabling "UnlockAllThings" gives you the cars every time the game starts even if they're prexisting in the garage
With "UnlockAllThings" enabled each time the player starts the game they'll be given the cars, even when the cars are preexisting in the garage. I found this out when suddenly my garage went from 5 cars to 15+ out of nowhere. With it disabled this doesn't happen.
That is just how the game is handling the cheat codes. It happens if you manually type UNLOCKALLTHINGS in the promo code and it gives you around 4 cars. You can controll the activation using F5 key or the F6 (check the script file). So its better to turn on the cheat when necessary or it will flood your garage witn 240sx, cobalt, dodge and pontiac.
But I like it! I sold these cars and got 100,000. It's like buying repair tokens and repairing cars. And grinding money is tedious.