Sure, they don't save energy in the sense that the laws of physics must still be obeyed- but by using the power of the catapult to increase the plane's velocity at takeoff, the motor can be optimized for flight rather than takeoff and less battery weight needs to be carried.
That's because catapults are used in places where space is at a premium. Catapult + full throttle will minimize takeoff length. But if space wasn't at a premium, then surely throttles could be reduced, no?