It is quite possible your dentist may be building a new bach, but that is not the reason you need a crown. Despite the rapid progress that has been made in the last decade regarding filling materials which can be placed directly in the mouth, these materials when placed in large cavities do not have the suitable properties and strength to withstand the chewing forces and forces of parafunction (Clenching, grinding). The biting forces exerted by the molars are between 390-900N which equates to 40 - 92Kg's of force.
Directly placed composite resin (white filling), is only suitable as a long term predictable filling, in teeth where the restoration size, as a rough guide, is less than 50% of the biting surface. Any restoration larger than 50% runs the risk of breaking prematurely resulting in a compromised restorative option. The choice however is up to the patient. Choosing the filling option in a tooth that is heavily broken down, commonly leads to the same filling been replaced 3-4 times in a ten year period, which ends up costing more than a crown would have cost in the first instance.
Teeth that have been diagnosed with cracks are best restored with a crown, or a restoration that covers the cusps of the teeth (onlay/overlay) to minimise further propagation of the fracture line. Teeth that have received root canal treatment are in a weakened state, and should be crowned to avoid fracture of the tooth, which would most likely result in loss of the tooth.