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It's a place of "refuge" for the tank. Not only does it add water volume to your system (which in turn creates a more stable system) it also will act as a natural filtration if/when you put in the proper algae. I won't ever do a tank (fresh or marine) again without one.
Also it can be a pretty cool spectacle if done right.
Most reefers keep some macroalgae in there which, helps remove nitrates and phosphates along with giving a safe "refuge" for copepods, Amphipods, etc to grow and keep the DT supplied with.