The main reasons to use fuel cells on electric vehicles are the addition of normal driving range and refueling convenience associated with the use of a chemical fuel loaded onto the vehicle as opposed to electricity. Note that when you refuel your car with gasoline, the power in the gasoline hose from the pump is about 15 to 21 megawatts!! That is the transfer rate necessary to load about a week's worth of driving onto a car in about three minutes. We do not expect to see the loading of batteries with electricity at such power (about 1000 times greater than the average home's electrical capacity).
Electric vehicles can have three levels of contribution from a fuel cell power system. Descriptions of the three follow.
The lowest level is, of course zero, with only batteries on the vehicle (battery electric vehicle)
The next level is the hybrid, range-extended electric vehicle, where the on-board fuel cell is used only to recharge the batteries on the vehicle. This would use a fuel cell a little larger than the average power necessary for the vehicle's drive cycles. This can be a few kW to a couple tens of kW. The benefit is that we no longer connect to the infrastructure with electricity, but rather load the vehicle with fuel, thereby benefiting with short refuel times and long ranges.
As fuel cells become less expensive, and compete with batteries on a $/kW basis, we can remove almost all the batteries and derive all our power from the fuel cell. This is the direct fuel cell powered vehicle configuration.