Summary
Device encryption provides an additional layer of protection against unauthorized access to data. BitLocker provides an easy way to encrypt a storage device in the event an attacker uses a method that bypasses permissions. BitLocker combined with the use of a TPM chip, protects the integrity of the startup process. When using BitLocker, a recovery key is generated. It's important this key isn't lost, otherwise that data will be lost in the event a scenario occurs that requires the key.
Encrypted File System provides an alternative to BitLocker. Instead of encrypting the entire volume, EFS encrypts individual files based on user accounts.