set_mode() is used internally to set the indicator mode for the different varieties of toggle button. There is no reason to use it unless you are creating your own widget.

This method toggles whether the toggle indicator is drawn or not. In GtkToggleButton itself, the indicator happens to take up the entire surface of the button, so that if the draw_indicator parameter is set to true the button itself vanishes. In the classes that inherit the GtkToggleButton methods the indicator takes up a very small area of the button, and setting the mode as false in those classes removes the indicator from the button, leaving only a GtkToggleButton with an offset label.