The container uses this information to arrange its child widgets and decide what size allocations to give them with Widget.SizeAllocate.
You can also call this method from an application, with some caveats. Most notably, getting a size request requieres the widget to be associated with a screen, because font information may be needed. Multihead-aware applications should keep this in mind.
Also remember that the size request is not necessarily the size a widget will actually be allocated.