Edit: After a realization I see you added them to the tab bar which I had hidden so I did not see it
A strange place to choose seeing as the native buttons are there as well with the tab bar enabled

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!