We've been wasting time playing Solitaire, Minesweeper, Hearts and FreeCell on our Windows PCs for decades, never knowing that those simple and addictive little games were secretly included to teach us stuff.
Solitaire was installed in Windows 3.0 back in 1990 to teach mouse-fluency with its simple click, drag and drop mechanic, and Minesweeper was put in to turn users into the ultimate code breaking machines.
Just kidding, it's meant to help users develop faster and more precise mouse movements and to make them comfortable with left and right button clicking. Beginning to see a pattern here?
Hearts was meant to get users comfortable with the idea of a local network and get people interested in Windows 3.1's networking capabilities.
And finally there's FreeCell, the game that wasn't meant to teach users anything- it was installed to test a data processing subsystem called a “thunking layer” which allowed 16-bit versions of Windows 3.1 to run 32-bit applications.
FreeCell and the other games would have been left out of subsequent versions of Windows if users didn't like them so much, but every time Microsoft leaves them out they get complaints.