My dentist has

First and foremost for strength. We age and the repairs often carried out on back teeth, for reasons of childhood decay, begin to break down. Teeth need to be strong enough to cope with forces of chewing and grinding. The best repair is often a crown.  It can be that strong.

Crowns can be made of metal for ultimate strength.  They can be made of metal with porcelain and are mostly appropriate for back teeth where strength and appearance are important. 

Pure porcelain teeth are mostly used in the front part of the mouth, where aesthetics are paramount and strength isn't necessarily an issue. 

A crown is needed to restore or brace a tooth having been broken down by decay or trauma.  A root canal treatment is required prior to having the crown made, if the decay or trauma has entered the pulp cavity.

The decision to crown is based on how much tooth structure is missing, the position in the mouth of the tooth, the long term prognosis of the tooth, the periodontal (surrounding) condition of the tooth, and many other factors.

A crown covers a tooth stump and restores it to its normal shape and size.  A crown can make a tooth stronger and improve its appearance.  It can be used to attach a bridge to it and protects weak teeth from breaking. 

A crown is a good way to cover teeth that are discoloured or badly shaped.  It is also used to cover a dental implant.  These decisions should be made jointly between you and your dentist.