Well 40% is enough volume for properly hearing what comes out of the headphones speakers. But I'm talking about using headphones with their own volume adjustment or external speakers. If you try to watch a movie on most laptops using their native speakers, you will definitely not going to hear a dialogue at 40%. You will want to make it play louder, but once you try to adjust it you'll be interrupted by this notification. I understand that people might need to realize that loud volume can damage their hearing, but I'm advocating that arbitrary checking of the system volume is not a proper way to do it.

I have some cheap unpowered Sennheiser headphones, and 20% in Windows is plenty loud enough for gaming. At full volume I can have them sitting on my desk and still hear everything clearly :D