There are many situations that would necessitate the need of placing a dental crown on a tooth. The most common reasons are

1. When a tooth has a large filling. For example when the size of the filling is larger than what is left of the tooth.
2. When a tooth is broken and a chunk of it is missing.
3. When a tooth is fractured or cracked.
4. When a tooth has had root canal treatment.
5. When a tooth is discoloured and for better aesthetic (if the veneer is not advisable)