I love carbon as well as gfo. In fw, carbon reduces algae, helps plants grow and cuts out odor. In sw, all the above plus removal of undesirable chemicals that might make their way in, not limited to, but including household cleaners, bugs that fall into the water, and things on hands, etc
Carbon does suck up plenty of beneficial things as well, but tbh, i have so many supplements which I add to the tank, that I have no problem with just keeeping my eye on the levels.