Navigation toolbar redraw button

What is the "Redraw the figure" Navigation toolbar button used for?
When I press it nothing (visible) happens.
I guess it may be for redrawing the figure (as it says!), but thats not needed
since (in GTK+) you setup an expose event which redraws the window automatically
when required.

It would be nice to have a button to redraw the figure AND undo any zooming
and panning you may have done. Which commands would I need to call to do that?