only1asterisk,
Knowing the max distance might help for when patterning a shotgun for home defense.
But I'd hate for anyone to think, "Well, the longest distance I'd ever fire at home is 20 feet, so that's all I'll ever practice." Practicing at longer distances really helps build trigger control, a crucial component of hitting where you aim at any distance. Even at closer distances, and especially under stress, trigger control should be so ingrained it is almost automatic. And one good way to do that is to practice slow shots at a distance.
(Yes, yes, of course you have to practice fast shots close up, too!)
pax