You CAN range a bit.....
with a duplex or #4, by establishing the DX the width of posts represent at 100, and the DX between pickets, etc, vs the known size of your target.
I'll not go into it, but its done.
Mil dots are better at it, again as long as you know the approximate size of your target, or an object near it. It involves some basic math which can be learned. Also mil-dots provide hold off points for windage and elevation in some instances.
If you are shooting known distances at any range, or distances say, under 300 yds, a duplex likely will suit you fine.