A tooth needs a crown when it no longer is strong enough to withstand normal chewing function on its own. There are multiple reasons a tooth could require a crown to function properly:
Large Decay: If a tooth has a large cavity that has destroyed its enamel and dentin, it will be too weak to hold a filling and chew properly. A crown can restore the tooth's functionality.
Large Fillings: When a filling covers more than half of the tooth, the remaining structure is too weak. A crown provides long-term protection.
Cracks: For tooth cracks that don't extend into the nerve, a crown prevents the crack from worsening by replacing the protective enamel layer.
Root Canal Treatment: After a root canal, teeth become brittle due to lack of nourishment. A crown prevents cracking and protects the expensive root canal treatment.