I don't remember the details, but IIRC grads are used in range finding. Something along the lines of:
- you know the tank (boom boom) is 20 feet long.
- in my binoculars (with a grad scale on the inside) the tank spans 10 grads.
- the tank is (20/10)x100 feet away.
I either read this in some Army field manual or maybe my father tried to explain it to me.