I have American and British rifles with both (or all three, if including the scope). It is common to see the standing express sight leaf filed down so one can see 'through' or 'over' it while sighting with the rear peep. I've often wondered how many of these were added after purchase and then mounted. Some do not have the leaf mounted and if you sight through the peep and through the standing leaf notch, you have a very precise way of aiming at a small point in the distance. My personal opinion is that the guys who ordered their rifles with both had that in mind. Most of these seem to have a folding peep so that the express sight can be used as the normal snap shooting sight.