Originally Posted by e4

Boiling cooks the starch so that it swells - like making gravy. It may still separate a little bit, but usually not much. This actually works better than just using cold starch - usually gives a smoother, stiffer finish and doesn't flake as much. Letting the fabric set for a while will allow the starch to penetrate better. Just remember, bugs like starch so you really shouldn't store fabrics pre-starched unless you know you are going to use them fairly soon.