I believe the theory is that hollowing 1) reduces weight of ornament so it hangs better(?); 2) hollowing reduces stresses on ornament, reducing chances of splitting, so therefore, the thinner the better, up to a point; 3) hollowing allows turner to control size of holes on top and bottom so you can get a tighter fit that will last longer because you let the globe dry thoroughly prior to making final hole cuts and gluing. There may be other reasons but I think this covers most important ones.