During the detailed engineering phase, prototyping is very important to identify risks that are not apparent in 3D CAD model. Tolerances and tolerances stackups are a good example of this – these can be tested fairly well with careful attention to detail, detailed calculations of what gaps and fits need to be incorporated.
But there are so many details associated with the definition of this design that a prototype and a few tests can confirm or deny instantly whether the desired goals have been achieved, whether it be a certain temperature reduction with a thermal management scheme, or that the force calculations you did are actually correct and the motor / gearbox size doesn’t need to increase before placing the order for 1000’s of them.