Solitaire was, is, and will continue to be the world's number one time waster and productivity killer, and Microsoft's decision to bring it back in Windows 10 is clearly good news for those who don't have anything better to do.
But the story behind Solitaire is much more interesting and has nothing to do with the idea of wasting our time when the boss is not around.
Solitaire was first introduced in Microsoft's operating system in 1990 when the company debuted Windows 3.0 and although it was a game, its purpose was not to entertain people or to help them relax.
Windows 3.0 was the first Windows version that put the focus so much on a mouse, so with Solitaire, Microsoft actually tried to help people learn how to use the mouse. In Solitaire, clicking and dragging cards around were the two things you had to do, so it was pretty much the easiest way to help Windows users discover the benefits of a mouse.
They used the drag and drop support without actually knowing it and just like MentalFloss writes, the fact that this particular feature is still around this days is proof enough that Microsoft's approach was very successful.
With Minesweeper, the story is very similar, but adds a little bit of extra details. While in Solitaire you only had to click and drag cards around, Minesweeper also introduced support for the right mouse click. So this time, you really discovered the benefits of the mouse. And what's more, it helped improve speed and mouse click accuracy, which at that time was something really difficult.
The games were still offered in the next Windows versions, not because people still had problems when trying to use a mouse, but because they became some sort of symbol for the operating system and everyone wanted to play the games every once in a while.
Eventually, Microsoft removed them in Windows 8 in an attempt to push users to the Modern UI. A touch-based version of Solitaire, which could also be played with a mouse, was offered in the Store, but since it was no longer pre-installed, its success was somewhat limited.
In Windows 10, Microsoft re-introduced Solitaire as a pre-installed game, but it's the very same version from the store with some improvements here and there.
And yet, with Windows 10 bringing so many improvements for the traditional mouse and keyboard, maybe the purpose is the same as before: to help people get used to the mouse after the Windows 8 disaster and the full focus on touch.