Royal icing

It's to prevent the icing going hard and also gives it a shine.

I don’t mind the fondant icing for a children’s birthday cake but not for my very special Christmas cake, so royal icing it is, with a little glycerine so that nobody requires an emergency dental appointment over the holidays. Of course I’ve already got plenty of glycerin in to make Delia’s truffle torte - anyone else remember the year when we couldn’t get glycerine anywhere because of this recipe?

The reason glycerine wasn't always used in Royal icing goes back to when Wedding cakes were tiered up with separate plaster pillars. If the icing was soft the pillars sank in and the cake tiers toppled. My mother worked decorating cakes for a company and her biggest fear was that a cake would fall.
They liked the icing to be like plaster of Paris and two layers were sometimes applied a few weeks apart before the decorating started in stages.

Glycerine acts as a catalyst, drawing moisture from the air into the icing. This I learned at a cake decorating course I did about 35 years ago and have always added it to royal icing mix.
Presumably the more moisture in the air, the less hard the icing becomes, if you have the cake stored under cover but not in an air tight tin.

Glycerin just keeps the icing soft so you don't need a chainsaw to cut the cake.

Yes,keeps it soft

Overthemoongran

I don’t mind the fondant icing for a children’s birthday cake but not for my very special Christmas cake, so royal icing it is, with a little glycerine so that nobody requires an emergency dental appointment over the holidays. Of course I’ve already got plenty of glycerin in to make Delia’s truffle torte - anyone else remember the year when we couldn’t get glycerine anywhere because of this recipe?

Yes, I remember searching for glycerine!

I didn't use glycerin when I iced my parents' Golden Wedding Aniversary cake and no-one complained.

I had bought fondant icing for my own wedding cake, and my sister and I swore that was both the first and the last time we would use it.

It was impossible to roll out decently!