This seems to skip over why this might be useful. I don't know either, but seeing how it literally is just an adjustable iris I'll make a wild guess and assume

This seems to skip over why this might be useful. I don't know either, but seeing how it literally is just an adjustable iris I'll make a wild guess and assume this is about depth of field. With the "external" iris close enough to the eye it mostly stops being a field stop (=blocking off the field of view) and starts to act like an aperture stop (=influences the aperture aka "f-stop" aka depth of field). I assume this is the explanation because for accurate shooting you have to bring the two thingies on the gun in line with the target, but the gun is quite close to you and the target is not. Lots of depth of field allows you to reduce the amount of blur you get on either. I'm guessing this is also why their posture is always with the gun extended as far away as possible, the closer the gun is to your face, the blurrier either the gun or the target gets. If you wear glasses you can test this yourself quite easily, punch a small hole into a piece of paper and take your glasses off. At an arm's length it's obviously a field stop, because you can only see a very narrow solid angle through the hole. If you put it basically on top of your eyelashes, it still reduces the field of view a bit, but you'll also notice how depth of field is increased.