Teeth have crown lengthening when a dentist either decides that he can not place a margin on a tooth for a crown without getting too close the the patients supporting bone(closer than 1.5 mm) or the tooth structure remaining is insufficiently strong to hold a crown predictably.
In the event that 'insufficient tooth structure' remains, removing bone aroung a tooth can expose new tooth for a future crown to grab onto.
Crown lengthening can be a good thing providing only a small amount of supporting bone needs to be removed and the final result is cosmetically appealing.
Crown lengthening is a bad idea if it involves removing supporting bone from adjacent teeth that might compromise their longtime prognosis, or if the resulting gingival position is unesthetic. In either of these two situations implants usually are a better alternative