Dentists wants patients to keep as many of their natural teeth as possible. That's why most dentists will fill a cavity instead of pulling an infected tooth.
Crown lengthening is a procedure that allows a dentist to save a tooth rather than pull it. The common procedure remodels the contour of the gum line. It doesn't actually lengthen the crown of the tooth, but rather lowers the gum line.
Crown lengthening may be the only option available when there isn't enough tooth structure for a crown to be placed on a tooth. On occasions when a tooth has been broken below the gum line, crown lengthening is often used to successfully expose more of the tooth structure, so the dentist has more tooth to work with when attaching the crown.