BCarp,
I am far from an expert, but my understanding is that the culpera is a fast growing algae which consume the nitrites (or traits??) in the tank for food. Also, they will compete with this food and starve other not so nice algae such as hair from growing in the main tank.
There is also an increase in oxygenation of the water from the plants which is a good thing.
A third benifit is it supposedly makes an ideal growing place for those pods, so you can move some live rock to and from the main tank to ensure a healthy population in the main tank.
Also, Pyro, from what I have read you can indeed put it in the main tank if you don't mind the looks. Its still benifical, and some fish will find it a tasty treat as well.
Any experts, please let me know if I missed anything.
-Scott