When we perform RCT in a tooth, the Pulp is removed, the Pulp contains Nerve & Blood vessels, and after that tooth becomes dead becomes weak.

Generally, pulp gives nutrition to the tooth, when we remove the pulp the nutrition of the tooth is stopped. So after RCT tooth becomes brittle & tooth can easily fracture.

At this time Dental Crown add support to the tooth which helps to protect the tooth from getting damaged. That way  After Root Canal Crown placement is very important.

A dental crown can strengthen the tooth, improving its overall appearance and alignment in the mouth. The dental crown is cemented into place on your tooth and it covers the visible portion of the tooth.  

Crown types which all commonly used: 

All Ceramic or Porcelain Crown: Zirconia or lithium disilicate-  has good strength, It the more esthetic, You can get a Tooth color matching crown. Zirconia crown is the most costly among the other dental crown.

Metals Crown: Gold- or silver-based alloys, titanium, or stainless steel-  has good strength, You cannot get a Tooth color matching crown 

Metal-ceramic or PFM Crown: porcelain fused to a metal framework- has good strength, You can get a Tooth color-matching crown.  

Composite resin, similar to that used in dental fillings: has poor strength than other crowns. It's the Cheapest among all dental crowns.