A sump isn't necessary. It's just a nice place to put all your equipment (skimmer, heater, pumps, etc.) and allows for some equipment that would be hard to implement without a sump (e.g. calcium reactor, probes for testing, bigger skimmers, etc.). It also increases water volume and it helps keep water level steady in the main tank (evaporation will affect the sump's water level, not the main tank's). That said, a tank can definitely be very successful without a sump.