I think what RS21 is after is the ability to swap to any car within your garage while on track. (single player arcade mode)
Therefore not having to leave the track come back a couple of screens, go to garage open the car list, select the new car, come back a couple of screens, go through the track select again and eventually get to drive the newly selected car!
I know you can set up favorites which can save a few screens, but this is still not ideal.
You can select any car while in a online room - why can't you do it in single player?
Perhaps a change of thread title might get you some more votes.