probably only as a sump, to hide your heater mainly, and add a couple more gallons to the capacity.
but at 55gal, you've got what, maybe 5-5.5 gals for that hang on fuge? I know my CPR hang on is right around 4 gallons, so its roughly 10% more capacity on my 40gal SW
but if you're gonna keep plants to help filter the main tank, I don't see a benefit to the fuge. if you're running a canister filter, you can get an inline heater to help reduce tank equipment clutter.
You could sell your fuge in our classifieds forum if you don't want it.