Sump help with few things. Sump make equipement quieter, increase oxygenation, increase mech filtration, increase bio filtration, you can put skimmer without having to listen to noise all the day.
Sump also increase the overall look of the aquarium: E.g. you put the heater inside the sump instead in the main show tank.
You can make more powerfull bio filtration and nitrates/phosphate removal with a good light and algae in the sump.
Sump also increase the overall volume of water your tank contains, so when a fish poop, it will be dissolved into more water, it make more room for mistakes, etc.
Sump is simply better for bigger systems.
I don't have sump on my hightech 20g, but I need to hear all the day the HOB