What is it?
A simple script of less than 150 lines of commented Ruby that use Inkscape to convert SVG files into all the sizes of bitmap (png) that you need. It was written primarily to help me complete the release of Fast Lists 2.0. Inkscape is a nice free (and Free) tool but the export UI made it annoying to create lots of different size bitmaps and I didn't know of a good way to do minor non-destructive changes to the shape of the page that you want to output keeping the drawing centred. This script makes it easy.
While I created it to help with iOS app icons and launch screens it can be used for any particular size outputs. When I use it I generate a couple of icons used within Fast Lists in addition to main App Icons and launch icons.
Why should I care?
Well if you have a designer to create every icon from scratch for each size (iPad, iPhone, portrait, landscape, retina, non-retina and for each context it is used in) then maybe you don't need this. But if you have to create the icons yourself generating them from Scalable Vector Files should produce better results than resizing bitmaps. This helps you do that process quickly.