Beside the nostalgic value, this hidden button has a very practical use when you use two monitors. I tend to have a window full-screened on each monitor. To close the window on the right monitor, I just mash my mouse pointer to the upper right corner (without having to aim precisely) and click. The pointer will "stick" to the upper right hand corner of the screen.

That is not possible for the window on the left monitor, because there is something to the right (the other monitor), thus, the pointer will not "stick" and it is harder to close the window (you have to actually aim).

Enter the "invisible button": For windows on the left monitor, just mash the mouse into the upper left corner and double click. No aiming necessary.

In Windows 10, most apps still have this button. But it seems to be tied to using the classic Win32 APIs or something. Modern "Metro" apps like the calculator do not have this invisible button any more. Let's hope that that style never catches on ...