When bacteria enters your tooth by either deep cavities, cracks, or older fillings, it can reach the nerve inside your tooth and cause an infection.  When the nerve of your tooth becomes infected a successful root canal lets you keep the tooth rather than having it removed.  If left, an infected nerve can cause severe pain, swelling, and the infection can spread which can be extremely painful and dangerous.  The infected nerve is cleaned, sealed, and filled therefore alleviating any pain.