A tooth needs a root canal when the space inside the tooth becomes infected or the nerve dies. This can occur as a result of tooth decay or fracture. If the nerve inside the tooth dies, then you are left with an infection filled space inside the tooth.. A root canal treatment is the cleaning and filling of this infected space inside the tooth where the nerve used to be. Modern techniques are used to preserve your natural tooth in place. This space is then sealed off to prevent new infections from occurring.