There’s no real reason to use a hard disk password. Encryption provides much more security and is more convenient to use. Skip your computer’s hard disk password feature and encrypt its hard drive if you actually want to protect your files.
Well, there is a reason. There is a performance hit with encrypted files that is noticeable with platters. With SSD, it's not noticeable, but then it will result in faster wear.
If you don't encrypt your drive at all, then it's trivial for someone to remove your drive, mount it, and read the data off of it. Your Windows account password doesn't protect the data.
Therefore, the hard disk password is a decent compromise. No performance hit with some security. An average thief is not going to do forensic analysis to get personal data off your drive.
You can think of this analogy:
HD password = locking front door
drive encryption = locking every file cabinet in your house
no HD password or encryption = unlocked front door