Widgets are double buffered by default; you can use this function to turn off the buffering. “Double buffered” simply means that Window.beginDrawFrame and Window.endDrawFrame are called automatically around expose events sent to the widget. Window.beginDrawFrame diverts all drawing to a widget's window to an offscreen buffer, and Window.endDrawFrame draws the buffer to the screen. The result is that users see the window update in one smooth step, and don’t see individual graphics primitives being rendered.
In very simple terms, double buffered widgets don’t flicker, so you would only use this function to turn off double buffering if you had special needs and really knew what you were doing.
Note: if you turn off double-buffering, you have to handle expose events, since even the clearing to the background color or pixmap will not happen automatically (as it is done in Window.beginDrawFrame).
In 3.10 GTK and GDK have been restructured for translucent drawing. Since then expose events for double-buffered widgets are culled into a single event to the toplevel GDK window. If you now unset double buffering, you will cause a separate rendering pass for every widget. This will likely cause rendering problems - in particular related to stacking - and usually increases rendering times significantly.