Active Desktop. That actually started with Windows 98, or at least that’s when it came bundled with. You had to install it on purpose on Windows 95 and NT4.

You could do some interesting tricks with this if you wrote your own local content for it. Different wallpaper images on different monitors, interactive wallpaper effects, and so forth. I have no idea what its actual intended use case was nor what anyone at Microsoft was smoking when they made this available by default. Parking anything on there that accessed external web content always struck me as rather a bad idea.