A crown is a separate procedure from a root canal.
Root canals are done to treat infection. Crowns are used to reinforce weak teeth.
There are times when you have a root canal and don't need a crown.
There are times when you have a crown and don't need a root canal.
Most of the time, a lot of tooth has been destroyed by a cavity that infects the nerve. The root canal takes care of the infection, and the crown takes care of the large amount of damage done by the cavity.