A sump is a tank under your display tank. It serves various purposes such as hiding unsightly equipment such as your heater, and also provides a larger water volumn to assist your bio-load. It has more uses but in a nutshell that's where it is the most advantagious.
It is my opinion that you do not NEED a sump. But you have to be very careful to understand and stick to the bioload that your tank can accomodate. If you have a 55 (I didn't look) then you have to keep your "inches of fish to a minimum" 1 inch per ten gallons is optimum without overkill.
That is not a lot of fish and applies more so to reefs. You can have more fish if you do not plan on keeping corals and you have plenty of live rock.
Yeah, I'm tired and I talk to much...sorry.