Root canal therapy is done to save a tooth that is severely infected. When tooth decay is large it can spread to the pulp chamber of the tooth. The pulp chamber and root canals hold the nerves of the tooth, which is why extreme pain is often felt when a tooth needs root canal therapy. Once the nerve is infected the only way to save the tooth is to remove the nerve and clean out the tooth by doing root canal therapy. Once this is done, the tooth becomes weakened and must have a crown placed over it to protect it.