What is use of Geometrical set in catia
They allow you to store any geometrical items ie, sketches, surfaces. it helps when you are trying to find specific items, but the most useful thing is that you can view the items in them no matter what body you have active at the time. i would advise storing all geometric items in theses rather than bodys.
Try to review this videos, maybe they explain your questions!! the link is in the following direction: