Mostly for doing animation - certain controller bones that doesn't affect to final result and yet it still exist on file. It could cause animation size unnecessarily bigger, and due to SMD it self have limited bone count, some bones might can left off from result - and I already have
few experience with this case.

To prevent this, we should add feature that exclude bones that we want.
I think to decide which bones that exclude from export, the bone group feature exist on blender might make it mostly comfortable.