You don’t need it at all; I don’t use it and have very clear water. It may help to take out odor or cloudy water, but I don’t know. I do know it is very useful to take out chemicals like medications. Other than that, I don’t know the benefits, if any, of using carbon on a daily basis.
I use active carbon in my canister (together with ceramic pipes, cotton balls, and filter pads), and I never had any trouble with the water.
It's not necessary to keep your tank healthy (although I do recommend everyone to have a little carbon around), but it doesn't hurt to use it next to other filter media (if you use blackwater extract or peat as filter media: keep in mind that it will remove the coloration from the water, and it might remove some of the nutrients those two media types add to the water as well).
Like others said, carbon is very useful to remove certain unwanted chemicals from the water (like medicines, after you have treated fish, or metals).