Activated carbon absorbs dissolved organics so it helps when you have a high bioload.
High amount of organics can produce ammonia/nitrite/nitrate. High organics could result in less oxygen in your water (This can be one of the reasons why my night time oxygen and ph levels are so low). High organics absorbs light which which is not good if you have photosynthetic animals. Some dissolved organics can also be toxic. Dissolved organics also smell bad (I put carbon in my skimmer collection cup and it works well to get rid of odour coming from it).
So if a tank has high bioload, these are some of the reasons to use carbon. Basically, if you already have a crystal clear tank and don't have any chemical warefare, you probably don't need to use it. For my tank, its a must.
http://www.reefkeeping.com/issues/2004-10/rhf/