3D prep is an advanced part of working with motion control since there is high expertise needed in working with real machines as well as with their virtual pendants in 3D software. On almost every job I remember was a need or at least a good use for blocking a 3D set with a virtual rig. When setting up a 3D set you´ll have the chance to look through a 3D camera with same settings as you´ll use later on shoot, and this is helpful a lot in most cases. You can find issues before rigging up the real set or react on things those go wrong doing that- weeks or days before director and DOP arrive.
With a properly measured and modeled 3D- rig, which behaves almost exactly as the real one in space, I can give very precise feedback to art department or lighting crew. In case there is a 3D preview of the upcoming shots and given to me as 3D scene, I can use those to constrain the MoCo rig to the camera and see what will happen on set. For that reason I made a fully IK- rigged Milo- and Talos previz tool for maya (MiloPrev), which gives exact response to the way of the rig through the set.
The upper image is a courtesy of Pantaleon Films. It´s a still out of "Der Nanny" by Pantaleon Films 2015, directed by Matthias Schweighöfer, DOP Bernhard Jasper