When is a sql server differential backup required?

A relatively small portion of the data in the database has changed since the last database backup. Differential database backups are most efficient if the same data is modified frequently.
You are using the Simple Recovery model, which does not permit Transaction log backups, and you want to perform backups more frequently than is practical for full database backups.
You are using the Full or Bulk-Logged Recovery model and want to minimize the time it takes to roll forward Transaction log backups when restoring a database.

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