Sumps add water volume which contributes to stability of the system. The falling of the water from the tank to the sump oxygenates it, and the overflow skims the surface of the water, which tends to accumulate dissolved organics, and interferes with gas exchange. With an in sump skimmer it also supplies this dirty water to the skimmer for processing.
A sump allows you to remove hardware from the tank such as heaters, which can burn some creatures. It provides a place to add additives to the tank in a non stressful way.
It also afford you the opportunity to create a refugium, which is very beneficial to just about any sort of tank. Probably a dozen other reasons people can think of.
In sump skimmers are much better. That's another one
That being said, there is something to be said for if it ain't broke don't fix it. If I were you, and my tank was doing great, I would look at adding in a refugium. If the skimmer is doing the job, might want to leave it.