Question

Why do potato chips manufacturers fill the packet of chips with nitrogen gas

Answer 1

Potato chip bags are not full of air, but of nitrogen gas. This is done to prevent the chips from oxidizing, which is part of what makes them go stale. Oxygen is very reactive, and likes to combine with other molecules whenever it can, causing chemical changes to take place.
As nitrogen gas is inert in nature so it does not itself react with the food items but prevent rancidity.