The silver is a layer of metal flim the metal doesn’t allow air to leak into the bag over time. It is this metal flim that make the bags idea for long term food storage.
Take a regular ballon fill it up with helium and take a mylar balloon and fill it at the same time. watch which one last longer.
Not really fair, the helium destroys organics like the latex, put helium in a mylar bag with food in it and watch what happens to the food.
The silver in the mylar is to add some strength to the material and make it formable or reflective depending on the amounts. Silver is one of the few anti-microbial that will kill any microbe and still is food service safe.
But if you are buying mylar just to get the benefit of silver, you can buy the silver in plastic and get the same affect without the cost of mylar.
I just can’t see the benefit from the mylar itself.