BitLocker protects against unauthorized access to the content of your internal or external drive.
Data on a stolen or lost computer is extremely vulnerable to illegitimate access. The person can get your data by running a software-attack tool, starting your computer from a bootable drive, or transferring your computer's hard drive to a different one.
However, BitLocker helps mitigate illegal data access by improving file and system protections. It encrypts the full disk, including all user files and system files on the operating system, and requires a password at login or the recovery key to unlock the startup disk.
Apart from that, the BitLocker To Go feature can encrypt the entire contents of an external drive.
Simply put, you don't need to worry about others stealing or leaking the information on your computer or external drive if it's encrypted with BitLocker, as the contents of the encrypted drive are unreadable without entering the correct recovery key.
Also read:
How to password protect, encrypt external hard drive on Mac?
How to encrypt, password protect USB drive for Mac & Windows PC usage?