mongorestore to --oplogReplay or not to --oplogReplay?
An OpLog is like a transaction log, thus if you had a 'full' backup of the system but wanted to get incremental data after the full backup was taken you can replay the opslog to get a 'point in time' restore of the database.  It let's you have more control on being able to address at exactly what point you want to restore everything.  Let's say someone dropped a critical collection at 12:05:23.  You could do a opslog replay up to 12:05:22 for example.