It is not necessary 100% of the time, but does serve a useful purpose. What alot of folks do is run carbon about one week of the month to help clarify/purify the water. Carbon can also remove good things from your water, so that is why it is not done 100% of the time. If you have corals, they can have little toxin battles with each other, the carbon helps remove things like this.
Filter floss just removes larger particles and can add to a nitrate count if not changed out often.