It saves the artillery guys having to do sums (always a good idea with artillery guys)

A radian is an angle where the length of the circumference = radius.
So a milliradian is one part in 1000.
So at 1km a mil is one metre across, similarly if you know an object is 10m long and you measure it as 1mil you know it is 10km away.

Most militaries round off the milliradians, to eg. 6400 so that it's easier to mark off on a compass.