The function of glycerine in fondant
Glycerine's functionality is a "moisturizer", in that it's very good at keeping moisture for a long period. In the case of fondant, it will help to keep it pliable for a longer period and help to minimize drying out too quickly when working with it (rolling out, icing sugar/cornstarch, application). It also slightly helps any sugars from crystallizing, which as you can guess, fondant has a LOT of sugar.

Note: It's the same glycerine in the cosmetic aisle.