To add a little more color to this, you had to be able to quickly repaint just a region of a window. And you don't have a bitmap of the window's current content

To add a little more color to this, you had to be able to quickly repaint just a region of a window. And you don't have a bitmap of the window's current contents because that takes up RAM and you'd need CPU time to update a copy every time the window updates (slowing down any animations). I don't know how it's handled these days, but when I was first playing around with Win32, you'd receive events telling you a region of your window was invalidated. You were expected to figure out what controls and images belonged in that region and just repaint those items because of the CPU load.