Cracked teeth are in danger of splitting completely which can be extremely painful and once a tooth has split, it usually cannot be repaired any longer.
A crown encircles the entire tooth and holds the fractured parts together even under biting pressure. This is the strongest and most reliable protection for a cracked tooth, especially since it can be very difficult to determine the true extent of the crack even using x-rays.
Other options include onlays which are partial crowns. Your dentist may select this if the crack clearly involves only a small portion of the tooth and if the tooth is reasonably intact. An onlay still requires that part of the tooth be trimmed, but less than would be removed for a full crown.
Tooth after being trimmed for a crown:
Crown cemented in place:
The onlay is attached to the tooth with a special glue.
Tooth prepared for an onlay:
Onlay glued into place:
Onlays depend much more heavily on the glue to keep it in place on the tooth than a crown. The glue may fail if there is decay around the onlay margins or if you have a nocturnal grinding habit.
Since the onlay does not encircle the entire tooth, it may be less reliable than a crown at holding the cracked portions of the tooth together. If the crack worsens, you may have to convert the onlay into a crown, especially if you end up needing root canal treatment.
