They are used to decorate cakes.

Of course a measuring cup. And other things like a pastry bag and a whisk.

with a piece of paper

Yes, real pastry bags are a lot more accurate than a simple bag with the tip cut off. They are inexpensive as well!

To make your own pastry bag you will need a zip top sandwich baggie. Cut a small piece of the one of the lower corners from the bag on an angle. Put the piping tip in the hole in the bag with the tip sticking out, and add the frosting.