The carbon absorbs oders, chemicals and such. The largest debate I have found to be is how long does it actually work before its saturated??? I can not answer this question and really don't think anybody else really knows either? Some manufacters suggest replacing every two weeks to a month???
I had always used carbon (charcoal), and in the past two years or so stopped using it all toghether. I have found no difference without, and it is the most exspensive of all the filter media's too. I spend very little $$$ on filter media, I just rinse it until sponge shrinks (6 months or more). I have a 100 gallon, 55 gallon, 20, gallon and just added a 35 hex to my collection. No carbon in any, water crystle clear, no oders either. Save carb to remove medications and such.
If your unsure it will not hurt anything and actually provides a great place for bacteria to thrive!
The most important thing of all, provide a home for bacteria such as your bio max, rinsing sponge only in aquarium water during partial water changes, alternating two sponges as noted in the other post is a great idea too. Bio max will never need replacing just rinse, I have been using these ceramic rings forever, just recently aquaclear came up with the idea of using this for their ac filter line. Have been made for canister filters for years, I always say anything made for a canister filter will fit in an aquaclear.
If you think you need extra filtration why not add another aquaclear , The internal filter will take away from your tanks appearence and internal space .