[tmz]

There are several purposes and some have several refugia.

Nutirent export can be accomplished by keeping a lighted refugium with chaetomorpha or other macro algae. As the algae grows it consumes excess nutrients and thus helps to prevent nuisance algae in your display tank. It can be harvested from time to time to give it room to grow. When you export it you are taking the nutrients it has absorbed out of your system.

Ph can be stablized to a degree by running the light on your refugium on a photo period opposite that of your tank. Doing so offsets the ph swing associatied with photosynthesis. Your algae will be producing oxygen when your corals are producing CO2 and vice versa.

A safe refuge for microfuana(zooplankton) is provided by a lighted or unlit refgium. This means you shouldn't put anything in it that will eat the zooplankton.