If I'm hunting turkey, a turkey home is in my gun. I know what my gun at choke will do from 10 to however far I feel my gun will ethically kill a turkey, in most cases, 40 yards. If he's closer, I know I have to be dead on. The 40 yard distance for patterning purposes just keeps everything comparable across the board. It makes it easy to evaluate from one gun/choke/shell, etc. to the next. Like when most people shoot rifles at 100 yards to evaluate/compare groupings. It's just a benchmark.