Think about all the stray electronic fields such as radio waves, cellular phones and many stray magnetic fields you can’t see. Guitar strings act like an antenna and pickup the various stray magnetic fields and when you touch the strings, the unwanted buzz is usually reduced as long as the strings are grounded to the controls. Your body effectively becomes a big shield. Having the strings grounded to the controls can increase the risk of electrical shock if you accidentally touch a system with reverse polarity.