the fuge is usually a calmer space for the pods and microfauna to reproduce. It also usually has a sandbed in it. If you have the drain from the tank going into the fuge, it will stir everything up and it will be a nitrate mess. ( When switching sumps or cleaning it out I've stirred up the sand and it created a nitrate spike in the main tank) The skimmer in the first chamber will not skim all of the water that comes from the tank. There is plenty of food that gets to the fuge. A lot of the water makes it past the skimmer.

I hope that helps.
Angie