I was looking up how to make my own powdered/confectioner/icing sugar. Some 'recipes' say that you should add a bit of cornstarch while others just leave this out.
So what is the role of cornstarch? Does it act like a filler (since it's cheaper than sugar)? Is it to prevent lumps? Does it help with texture? Does it do something else?
If this question is too broad, assume I'm only talking about frosting, since that's a frequent use of this sugar.