It prevents the water from splashing as it falls into the tank or sump. This splashing causes drops of water to go all over, onto your lights and the side of your tank. That water then evaporates and leaves behind the salt, just like it does in your tank and requires you to top off. Unlike your tank, there isn't a large water source around your lights to absorb that salt, so it stays where it lands. That is your salt creep. Reduce/remove splashing you then reduce/remove salt creep. Nice thing about panty hose is that it will also catch large debris that is in your water so it also acts as a cheap filter.