Principle of least privilege for sql server agent jobs
Maybe the wording is unclear, but least privilege applies to all activities and should never be ignored. 

The least privilege you need to perform database maintenance is sysadmin. Lower privileged users could perform those activities too (I'm thinking of db_owners), but the least privilege needed to perform any activity on a SQL Server instance is sysadmin, so if you have a job that performs administration tasks, it makes sense to have it running as a sysadmin.

For tasks other than database maintenance (tasks that implement some business logic), you want to impersonate users that have permissions to perform specific activities on the database.