Several of our automated spec tests (e.g. FLE, retryable reads) require use of the "admin" database for CRUD operations. 

In practice, we generally don't recommend that applications store user data should in the admin database, and so by using the admin db in tests we end up not testing the behavior that actual applications would experience. In particular, in sharded clusters the admin database is actually located on the config servers, not one of the shards.  

See SERVER-42842 for one example of where using the admin database inadvertently broke driver tests on sharded clusters.