Just like a visual field test at the optometrist or any other eye exam, the blinders help you focus with one eye. You can't trust both of them together, so your eye doctor covers one up to allow the other to do the hard work of focusing on the target, testing one against the other. Olympic shooters who use these want to cover one eye with blinders, or darken it so that they can focus with the other eye more readily.
Additionally, the mechanical irises contract and expand depending on how much light you want in your eyes, just like the aperture on a camera lens. On any camera -- most apparent on those DSLR cameras with detachable lenses -- using lenses with larger apertures, like f/1.4 or f/2.8, will result in soft-focus backgrounds (or shallow depth-of-field). Sometimes a photographer or cinematographer might want to create that effect, with a face or a subject in sharp focus and the background a wash of haze. Opening your aperture that wide will also result in a much brighter field of view.
The opposite is true for a sharpshooter. The smaller the aperture, the deeper the depth of field, and the farther the sharpshooter can see with accuracy. As the Wired story explains, they want both their gun sights and their target in as sharp a focus as possible immediately within the brief seconds they have to aim, pull the trigger, and hit the target.
Some fools have called them "incredibly dorky," but we're willing to bet they haven't won any gold medals. That said, they could cost you upwards of $500.
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