I don't even really get why the russians made it adjustable to out past say 600 meters, never mind 2000, did they really expect soldiers to be using iron sights out to 2000m?
It was not for precision aimed fire. Like many rifles of the late 1800s, the Mosin Nagant rifle was designed to be used to stop enemy troop advancements. As such, it was supposed to be used for "volley fire", to fire on enemy troop formations from as far as you could see them, often a mile or more away, taking out as many as you could to reduce the number of troops charging your position. A good idea at the time, sort of. Watching your comrades falling as you advance towards the enemy has a demoralizing effect, and it reduces the number of troops that will need to be repelled with bayonets. Since the early 1900s, this role is filled by machine guns, mortars, and light artillery.