You should use a non-standard read operation when you want to read a subset of the attributes on a database record, or you want to use a key other than the primary key of the entity. Non-standard operations use a key that you specify to locate the target record. It is not possible to guarantee at development time that only one record will be targeted. If there is more than one record in the result set, a runtime errors is generated.
Non-standard read operations can be more efficient than standard ones because they result in less database I/O.
As with any operation where you specify the key yourself, there is no guarantee that the database will be able to access the target records efficiently - it is up to the designer to ensure that appropriate indices are defined to ensure this.