Use case
Quite often you want to split an object exactly along a given flat surface, with all part of the surface in object A and the rest becoming object B.
This currently requires you to take measurements about the angle of the surface, and th very slowly and carefully align the angle of the cut feature to that, and then to also move it to the right elevation.
That is needlessly difficult, especially for surfaces that are at odd angles.
Solution
Just have an option to click on a surface of the object to align the cut tool to cut at exactly that surface.