I am planning to build a omnidirectional holonomic robot, and checking what I should use for the hardware, I saw many people using a Raspberry to compute most stuff, which in turn calls an Arduino to control the motors. But since the Arduino will have to drive some H-bridges, and the board computer is probably a lot more powerful, it seems to me that those drivers for the motors could be controlled by the Raspberry-ish board.
What am I missing? Why so many people use both at the same time?