what is its purpose? is it really needed for a carburated car?

not needed for low pressure pump with no return line.

actually it is needed to prevent over flow of tank (or over presurization) this can happen with a near full tank, on a cold night that turns into a hot day, the gas expands a god deal, and must have some place to go.

the expansion tank or a vent is needed a vent can cause a fire hazard in a garage with pilot lights, so the expansion tank is really the best bet.

carberated or fuel injected, it does not matter, you should have an expansion tank.

I will most definitely use the carbon canister and expansion tank on my 2056cc