Dentists crown teeth for a number of reasons including; to protect weakened tooth structure, esthetics and also to open bites. The first reason is by far the most frequent. Teeth that are weakened by removal of significant tooth structure are more prone to fracture. When restoring a weakened tooth dentists often recommend either a crown or onlay, since these are better at distributing vertical forces on these teeth.
Often patients will ask their dentist to place a large filling rather than a crown because they feel this will be a more conservative and a more affordable restoration. Unfortunately large multisurface fillings are hard to place and probably need to be replaced more often than smaller more conservative fillings. Also since large fillings allow greater cuspal flexture they can make it more likely for teeth to develop cracks that can compromise the life expectancy of a tooth.