As long as you have chaeto or something and run floss/filter socks that you regularly remove/clean, you don't really need carbon. But if something kicks the bucket or releases toxins, carbon could keep the tank from collapsing and does indeed remove many decomp results/toxins.
You don't need it if you keep up with water changes, but there's no doubt that it does improve water quality and is definitely helpful when housing things such as leathers, some sea cucs, and w/e corals engage in harmful alleopathy.