Numerous reasons... Root canal treatment may be recommended to treat deep decay, to remove infected or severely inflamed tissues within the tooth, or to allow a post to be placed for crown preparation.
Damaged Nerve. Irreversible nerve damage can only be treated by removing the damaged tissue and sealing the canals with a root canal or by removing the tooth.
Pain/infection. Bacteria in a tooth's nerve from decay or fracture will lead to inflammation of the nerve and eventual infection. Once it is determined that a nerve is infected or will not get better, a root canal is needed. Pain is not always present. Some teeth may need root canals to allow for a post that can anchor fillings or crowns on to the tooth.