carbon is used to seperate you from your $$$.
and a bunch of other reasons already mentioned.
i, personally, have not specifically used it in months. my emps come with it included in the cartridges but i rinse and reuse these constantly. ditto for the whisper type filter bags, if it comes with the carbon i use it, but after that i just rinse and reuse the filters until i can do it no longer. i do have a big, brand new jar of it i use for a paperweight tho.
it can be used for eliminating oders and some organics i believe, but regular maintenance works better and costs alot less.