That plus release of O2.
The fluctuation of CO2 and Oxygen between the day/night cycle is minimised. And thus, pH is maintained / balanced between night and day.
Some people encourage algae to grow in the refugium sump, where the algae will take up the excess nutrients and nitrates. So the sump can look as nasty as it needs to be since it's hidden, while the tank looks clean and free of algae. This is making algae work to your advantage.
Just to avoid further confusion, we should call it by it's correct name.
Sump = no light.
Refugium = with light.
Some other benefits include:
A great place to grow nitrate eating plants in a Cichlid tank. The cichlids would destroy the plants in a tank otherwise. But in a refugium, the plants can thrive and help with the intake of nutrients.
An alternative to housing fry or shrimplets in breeding boxes, without impacting the aesthetics of the main tank.
Hide all the unsightly heaters, skimmers, etc.
uh, probably something else I have forgotten