With a sump you don't have any unsightly things hanging off the back of the tank.
My sump has EVERYTHING in it. This includes the heater, skimmer, doser, chemicals, etc...
Another plus for a sump is that it adds more water volume to the tank. Typically, the more water volume you have, the more stable your water parameters will be.
But you don't need a sump. Its kinda an extra perk.
Can I do a reef set up with just a dual hang on skimmer? thats the only thing I have to worry about right?
Equipment wise, you'll need a power head (or more depending on tank size) and an appropriate sized heater. You'll also need to decide on a filtration method. The Berlin method of filtration is primarily used for reef tanks. You'll need either a DSB
or a refugium as well. Protein skimmers are optional, but recommended.