The main reason for cars is the added realism as most households have cars and since players see cars in the background it makes them want them as actual possession. That said yeah they'd serve no purpose other then to look at.
Ts3 cars made sense as different cars had different speeds so the better the car was the faster you'd get to places but ts4 has loading screens so for traveling they have no use. The only thing I can see them being useful for is if you could work on cars to build your handiness skill but again once done it would useless other then maybe a new woohoo spot and to open up your own car dealership if you own gtw.
The only way I could see cars being useful is if they restricted world to world travel by the use of cars. If you didn't have a car your sims would be restricted to the world they live in but with a car you could go to other worlds, either that or make so that if you didn't have a car your sim had to call a taxi and pay to travel to other worlds. All this would be fine if cars were in the game from the start and honestly would make sense but changing it now would be restricting us and adding in a uneeded feature just to give an object use.
Honestly I'd much prefer bikes and skateboards not to travel with but just to have my sims ride/skate around with like how they can go and jog around a hood. Also with bikes and skateboards they could add in skateparks so are sims could perform tricks and maybe compete in competition. We could make famous skateboarders or bikers.
EDIT: Now I'm not saying I don't want cars cause I do and I'd add them to my sims houses or at least to the ones who I think would own a car but I can continue without.