Refugiums have 2 main purposes that I can think of:
1. Nutrient export, translated into english is removing excess nutrients, and what I call fertilizer. As the tabk accumulates waste product, macro algae can consume it, therefore exporting it. They also provide oxygen. Things that I have had trouble with are nitrates and phosphates, both of which are in some measure used by plants. These can be harmful to fish , inverts, etc.
2. Refugiums also are used to grow macro algae which feeds tank inhabitants. Some fish eat it etc. Coral and anemones also benefit from the good nutrients produced in the refugium.
Some reefers will also use the refugium to house fish that are stressed out, or "bad" hitch-hikers that come in on live rock. There's alot of info. out about refugiums so you shouldn't have any problem figuring out if they can help your system.--J