Why a skimmer if you have a overflow?

Not a silly question. Willing to bet most have asked the same at some point. The overflow box only skims the scum off the waters surface. A protein skimmer removes nutrients from your tank by basically stripping the water with air bubbles. Surface skimming itself is not a form of filtration.

What the overflow does is control the water level in the tank. The return pump from the sump may pump at different rates, but what goes into the overflow will allways match what the pump is returning to the tank. So the level in the tank allways stays the same.

A Protien Skimmer, like Greech said, actually removes organic material from the water using a combination of tiny air bubbles and Voodoo, and so reduces the ammonia and nitrate going into the system

Ian