Mildots were intended for ranging. Know the size of something but not the distance measure it in mil and half mils and you can calculate the distance. Conversely if you know the distance you can calculate the size.
Once you are familiar with it you can estimate distance much quicker and with sufficent accuracy out to around 4 to 500 metres to connect without the time penalty of going for a rangefinder then back to the rifle.
It's not as accurate obviously but is good enough in many situations and has the bonus of giving you your holdovers without the need for knob twiddling.
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