So what’s all that stuff for?  The gelatin, the glucose, the
glycerine, and the fat you mean? What purpose does it serve in a
batch of rolled fondant? Obviously it keeps it flexible, but
then water will keep a mass of powdered sugar flexible too…at
least for a while. Why all the fancy stuff?  The first part of
the answer you can probably guess for yourself. Powdered sugar
is made up of sugar crystals. Crowd them together in a paste and
they’ll want to form even bigger sugar crystals, the urge is
just too strong. You don’t want your fondant icing forming an
impenetrable force field around your cake of course, so steps
must be taken to keep those little sugar LEGO’s from connecting
with one another. Gelatin is a good start. Those long, ropey
molecules will wrap themselves around some of the crystals,
preventing them from getting a tight grip on one another
(they’ll also give the fondant a little “body”). All those
little ball-bearing-like glucose molecules will also help in
that regard, flowing around and in be