In windows 10, especially on the Surface devices, the ability to turn off the screen has been disabled. The old method was to press windows+p to get to projector options, then arrow up to second screen only and press enter.
There are three reasons why having a way to turn off the screen is important:
1. Privacy and safety. As a blind user, we have no idea who could be looking at our screen and if we are writing some highly personal data in a public place, then we have no way to insure someone is not looking over our shoulder and reading that data.
2. Eye fatigue. As a nearly totally blind user, my eyes start to hurt if my computer screen is the only light in the room.
3. Battery life. The screen is one of the most power intensive components on my computer and in Windows 7, I would get an extra 1-2 hours of life compared to the dimmest brightness setting.