It is a belt and suspenders approach by using two pumps - if one goes out or you want to clean one, you still have one for circulation.
There are those that suggest having too much flow through the sump reduces the effectiveness of the skimmer since it doesn't have as much time to process as much water before it is return to the tank. More important to have more flow in the tank such as with power heads than rely on the sump pumps for flow. I run a pump in the sump once a week to stir up the detritus for the skimmer to take out.
I would rather have my SWID pump take water from the main tank rather than the sump to reduce the sump turnover (and noise, which isn't too bad as it is) but haven't found a good way to do it w/o putting pvc into the viewing area. Don't know that I need the SWID in addition to a Koralia 3 in each from corner and a Koralia 1 facing the front glass, but I have it and the pump so why not go ahead and use it? I considered putting it into the overflow but think that the pump will pump the water out faster that it will go in.