The horizontal-viewport-segments query targets the device when it is being held like a book and the two screens are side-by-side.

And vertical-viewport-segments targets the device when the two screens are stacked on top of each other and fold like a laptop. I've found this mode useful when using my Surface Duo with PowerPoint. I can access my speaker notes on the bottom screen and my presentation is on the top screen. Pretty snazzy.

— Designing for Dual Screen and Foldable Devices With CSS - Stephanie Stimac's Blog

With the advent of foldable devices (which I predict will last for only 5 years), it is important to know how to develop against them. It never crossed my mind that you might want a webpage that is aware of the fold, but if you want a web-only version of powerpoint (or gameboy emulator), it totally makes sense.