it's a dust cover, which is often a feature for the type of welding goggles it's based on

I do not at all understand why on earth there is a metal lid thing on the top, what use could it possibly serve, and why is it just a rectangle, it's not the same shape as the goggles

I'm always a fan of simple, clean cosmetics like this. Too often people try to make centrepieces that require building a whole loadout around, instead of something versatile that can work in all sorts of sets.