motorsaver?

That part number does not come up on ay of Siemens' sites here in the US, but if it is just a molded case circuit breaker, the reason is that MCCBs are not motor overload protective devices, only short circuit.

The MotorSaver is a solid state overload relay that has several other protection functions built-in, making it a somewhat sophisticated motor protection relay. Its like saying why not just wear a life vest istead of getting in a boat. Both are necessary, but one without the other is risky.