It's a different type of security:

I leave my email, jira, slack, and other services logged in while I am working. When I step out of my computer I lock my screen to prevent other people in the room from using my account in those services.

I am aware that they can grab the hard drive and get whatever crap I've got there, but they cannot login into my email, bank, and other services. That's why the lock screen is useful for me.

Windows can definitely also do full disk encryption via BitLocker...

It couldn't when a lock screen was introduced in the 90s. Bitlocker being enabled by default is a pretty recent thing.