Constant video playback isn’t representative of regular use. Who is going to sit down with a laptop and watch 16 hours of non-stop video without doing anything else?
Manufacturers don’t care whether this represents your experience or not. They use this benchmark because it produces the longest battery life.
Modern laptops (and smartphones) use hardware-accelerated video decoding. The laptop has special hardware in its graphics processor unit (GPU) that efficiently decodes the video while using as little power as possible, keeping CPU usage down. For example, when you play an MP4 video—even if it’s on a website or in an app—this kicks into gear and saves you power.
This is a great feature. It helps save battery life and keeps your laptop (or smartphone) running cool while you watch videos. However, manufacturers abuse it by using this number to boast about battery life. Anything else—whether it’s browsing a single website or just writing a document in Microsoft Word—will use more battery power than playing videos.
PC manufacturers often benchmark battery life using Windows 10’s included Movies & TV app, which is always set up to use the PC’s hardware acceleration, if available. This feature is not available in every video player, and may not always be enabled by default if it is.