Teeth sometimes have large portions missing due to decay, fracture, the loss of a filling and, in the case of root canal treatment. In such cases post & core is required

A post and core is a type of dental restoration used either to stabilize a weakened tooth or provide an anchor for a crown. Utilized when inadequate tooth structure remains to support a traditional restoration, it consists of the insertion of a small rod (known as a "post") into the root space of an affected tooth, leaving several millimeters protruding. The protrusion is then used to support a large filling, or anchor a crown.