Reduction of Friction

You can save energy otherwise transformed to heat.

So it's not as painful.

Reduce damage as well
You can also prevent extensive damage. For example, oil in you car is used to reduce friction between moving parts. Take the oil out, the car stops running in short order and you need a new engine.

But sometimes friction is desparately needed, imagine how to start a car without friction?

Friction with the road allows the car's wheels to work, and friction with the road allows the car to turn. When a car goes in a circle, friction supplies the centripetal force necessary to keep it from flying off.

wouldn't a car actually go even if there wasn't friction because of newton's laws? There is exhaust going out the back after all...

if the exhaust were pointed thata way--yes. But the direction of exhaust is at the discression of the muffler builder---didn't ja ever watch Orange County Choppers--sum of dem bikes would be going straight down.

Rap

But wouldn't the horizontal component of the muffler's exhaust vector make it go? Unless the line that the muffler lies on has a perfectly perpendicular intersection with the ground.

on a two wheeled chopper, the specific impulse of any exhaust that was slightly off center would make the bike fall. There's would be no friction to prevent lateral slippage of the tyres.

So 'Sis Boom Baah Micky's gonna fall.'

Rap

I don't think you'd get much thrust from a car exhaust.

Agreed--but we're arguing in the fictional world of no friction. So the pressure of the exhaust gasses at the manifold would be the same as that at the exhaust tip-less diffusion as the result of expansion. So the impulse force could be as much as the product of the exhaust pressure and the area of the exhaust.

Of course this is contingent upon direction since this force would be a vector and the wheels would only merrily spin or slide.

Rap