Mainly used on longhaul to note crossing of Lat or Long. If you asked to call another OAC at, say 45 north, you would enter N45/1/1 which would put a waypoint along your track line at 45 north. If you wanted multiple points for plotting or other reasons, then increase the frequency or distance between the points.
Next time you are airborne, try it. Make sure the secondary is copied - it is very easy and once you have seen it, it becomes a useful tool.