It is very usefull under KDE and other window managers systems. Obviously not in Windows or MacOS.

useless

Exactly what I was looking for. I use it on Xubuntu (with XFCE). I configured my system so that there is no titlebar in maximized windows. I put the Minimize button into the tab bar. So I can still minimize the window with ease.

Great!

It does what it says... But why? I mean what's the point?
It sits right next to a button that does exactly the same.

It will add minimize botton to the toolbar,great!