Helps keep phosphate down
Helps keep nitrates down
A place to store a few extra pieces of live rock that you just don't want in the aquarium at the moment, but might use later on
A place to keep a fish that is being picked on in the main tank, especially if you don't have a quarantine tank
A place to harbor pods without the threat of the many fish and inverts in your main tank
Helps keep pH stable
Depending on lighting and how it's set up, it can double as a frag tank (mine does)
Depending on your fish, you can grow certain algae and harvest them to feed to your fish instead of always having to buy sheets
Helps to oxygenate your water (not a huge factor, but it's still there)
I'm sure people can add to this