well if you don't have a skimmer than it will help you with you bioload's filtration.it will help to maintain the water crystal clear.. carbon is not important in reefs unless that's part of your main filtration. but if you got a skimmer and keep up water changes there's no need for carbon. it's just some xtra support..
also somepeople do fine without a skimmer or carbon(in nanos)filter floss is fine, just keep it clean .
so carbon it's just an extra support on removing phosphates,nitrates and other dead organics. but if you have that extra support, you'll have to change it often and maintain it just like the filter floss...