A sump has many uses, some also have the sump on display.
1) Adds water volume to your system
2) Can house all the equipment, yes inc heater
3) Vastly improves airiation, esp where the sump is a few feet below the display tank.
4) When you mess with kit your not disturbing your fishes.
5) Evapouration does not lower the water in your tank, only the sump!
6) When adding meds, water changes etc you can do it in your sump giving it time to mix with existing tank water (less shock for the fish).
7) It can be used as a refuge for scavenger fish like ghost shrimp. Handy if you cant do that in the display tank owing to your fish eating them.
8) Can be used as temporary home for a weak fish or a bully fish to cool off for that matter, big advantage here as you are seperating the fish while keeping it in the same water, hence no shock.
Is that enough to be going with?