CMB is mainly used for zooming and rotating a 3D-model. (as long as you don't use the 3D-mouse for that)
Additionally, CMB can reduce workload in other situations.
Pressing CMB in a feature dialog is equivalent to clicking the OK button. Therefore you save the time spent for navigating to the OK button.
An additional advantage is the fast completion of steps within a menu, like selecting multiple curves after one another. E.g. when defining contraints within a Sketch (explained in chapter 4.1.1), you may have to select multiple curves. For that it is useful not to drag the curser from the curve to the menu point back and forth. Instead, you can click the CMB to go to the next menu point and just focus on selecting curves.