I use carbon to reduce the effects of chemical warfare from the softies. The hard corals seem a bit happier now that I'm doing it. They stopped expanding quite as much (maybe) when the sarcophyton got big.
The reason you might not want to use just any old carbon is because some leach PO4. The expensive reef types guarantee they don't. If you buy the cheap stuff, put some into some seawater and test the water for PO4.