When tooth decay is deep within the tooth but not affecting/getting too close to the nerve, you want to make sure it remains that way. In most cases, too much of the tooth structure has been compromised. At this point, if the tooth were to be filled, the risk of breaking any out part of the tooth would be too great, ultimately resulting in total tooth loss.
As most of the original tooth structure has been removed, the structure needs to be strengthened with a “build up”, a procedure that provides a firm foundation for the new crown. A porcelain crown is then made to protect the tooth for years to come.