They are an additional place for biological colonies to grow. In general they are a benefit. If you have a very small bioload then they are probably a waste as the bio-bugs have plenty of other places to grow. If you have a higher bioload they will be of more benefit, allowing for more of the beneficial bacteria to grow. I probably don't NEED them but I am using them as extra bio-media that can be used to jump start cycles in my new tank(s). Also, it doesn't hurt to have more filtration than absolutely needed.
\Dan
