If you're talking about info from Battery University, I've heard that their articles aren't exactly reliable. I used to cite them as well. They're also old, and written about Lithium Ion batteries far more outdated than those in modern consumer electronics.
Besides that, operating systems have a built-in power-down function that prevents the batteries from fully discharging to protect their health. I don't have a source for this other than experience with various devices. Take your phone, for example. If an Android phone "dies", it powers down. You can actually turn it right back on many times after this - but as soon as the OS boots up, it will immediately say "shutting down". The battery still has power in there, but the OS displays it as "0%" to indicate that it's powering down to prevent damage to the battery.
Same principle roughly applies to Surface devices, Apple devices, etc. You can still do things that require power, but they won't boot. This is to protect the battery. I can't figure any other reason, can you?
So, these batteries always have a certain percentage left. Feel free to leave some charge to extend longevity, and keep it under ~80%, but the battery already has certain protections in place. (I've heard 20-80 as the key ranges).