in simple terms, when you move around you can accumulate an electrical charge. when you touch something conductive like a metal case or circuit board, the charge dissipates, which essentially means that electrons are flowing from you through the object that you have touched. if you touch a very tiny wire on a circuit board, you can send a large number of electrons through it in a very short time period. too many, in fact, and it will destroy the wire, and oftentimes your component. by touching the metal case you allow the electrons to dissipate throughout the large volume of metal, instead of a tiny wire on an expensive component.