In the UK, it used for the “high holiday” cakes — Christmas and weddings. In North America, it is used to make decorations such as flowers, etc., for cakes.
It’s too stiff to apply to cakes directly, as it would tear up the surface of the cake. That’s why marzipan is put on the cake first as an intermediary layer. The icing sets hard enough to support easily another cake on top of it, making it ideal for tiered cakes.
In the UK, you can buy Royal Icing at any supermarket in block form ready to use, You roll it out, cut to fit and press on over top the marzipan (which you also buy ready to go.)
Its hardness makes it ideal to use on baked goods that have to make trips in the back of cars. It is very resistant to cracks, and it won’t subside on you or slide over to one side of the cake.
Royal Icing is very hard to mix by hand. An electric mixer comes in handy here, preferably a stand mixer, but don’t whip air bubbles into it.
If the Royal Icing hardens around the edges of the bowl while you are working, scrape those hard bits off and discard them. Don’t stir them back in. If you find it’s drying out on you while you are applying it, cover the bowl with plastic wrap while you are working.
To test consistency, lift a bit on a spatula or spoon and let it dribble back into the bowl: if right, the dribblings should stay on the surface for about 5 seconds before being absorbed back into the whole. When you have this consistency, it is good to use for coating things with.
For other purposes, such as piping or “gluing” cookies together, you’ll want a stiffer icing; the dribblings should last for about 10 seconds.
To thin, use a bit of water, egg white or lemon juice.