A big one: no need to find parking! Or to go back to your parked car on the reverse trip. You can do a triangle trip A,B,C,A with public transit. With a car it has to be A,B,C,B,A as you left your car at B.
If you're sprinkling in little rules like "ah but you couldn't park at C", well, "the bus doesn't go to C" is also a valid restriction I run into often. So often when I take transit, its drive to train station -> take train someplace -> take bus someplace -> walk several blocks to destination -> walk several blocks back to bus stop -> ride bus to train station -> ride train back to train stop -> take car home so a A,B,C,D,C,B,A trip is actually quite common just to do the same as A,D,A with a car.