Crowns are used to protect a tooth when the remaining tooth structure is not strong enough to be supported by just a filling. Because a tooth becomes weaker once a nerve is removed, it is important to crown a tooth after a root canal has been performed. This helps to prevent the tooth from fracturing.
Crowns are also often used for repair of fractured chipped teeth where a standard filling will not work.
It is important to note that once a filling or crown has lost its seal, a cavity will quickly form under it. A cavity in the area of a restoration is already past the first line of defense of the tooth. Waiting may allow the decay to reach the nerve. The only treatment option once it reaches the nerve is a root canal. For these reasons, it is important to deal quickly with a loose or broken filling or crown.