Actually, the Large aperture is for night and Tony is 100% correct. but and now try and follow me. There is a SWEET SPOT inside BOTH apertures. If you look carefully at the little sight there is a real small pinpoint of clear area, if you set that sweet spot on the front sight, the front sight will become crystal clear, with little or no concentration from YOU the shooter. Now look at the Larger aperture and you will notice that the aperture itself is NOT Centered and is actually off centered just a whisker. It too has a sweet spot and by using the Long Range / Low Light your Elevation will be affected by two clicks. Now for you EXPERIENCED shooters, you should already know about the Sweet spot, but if not, hold the muzzle down towards the ground and then hold your head about two feet from the sight until you can see it, now as you bring your eye to the spot you shoot from, hopefully with your Nose smashed against the Charging Handle, you will see the spot and you only have to put it on the front sight and follow your other procedures from there.
We flipped the RS to LR/LL only if we were having to do a quick change, Rattle Battle, Sun went behind a cloud etc and mostly ONLY on the older A1s, for changing Elevation was ONLY on the Front sight with a Nail and there was no means to adjust elevation from the rear. With the A2s it is now up to the disgression of the shooter whether they Flip or Adjust, I never used it after we got rid of the M16A1s and went with at least A2 Uppers.
But the LR/LL is a 2 click change, for most shooters, regardless.