In the beginning, Windows was remarkably flat.
Visually, that is.
It was hard to tell what could or couldn't be clicked on, as the screen lacked visual clues to help guide you. This was a particular problem for buttons, toolbars and other controls.
(I could insert a snarky comment here about repeating the mistakes of history...)
Then came CTL3DV2.DLL.
It changed everything.
Windows 3.0 was the first version to get some real 3D going, and Windows 3.1 continued the trend.