Ok, for the typical hunter who just runs his gun with whatever shells are cheap and made by someone good and pretty much only hunts upland game and doves, maybe the occasional duck or goose, there's not much use in patterning a shotgun, now, when it comes to guys that turkey hunt, it's a LOT more important-when you take your shot count down from 400 to 9-100, you're really changing things on the other end. A shotgun is basically just the same as a fixed sight handgun, there are no guarantees that it shoot to POA (point of aim) out of the box. When you've got a 6' spread at your hunting range and 400 shot in there, you've got some room for inaccuracy (your precision may be top notch, your center mass of shot (CPOI=central point of impact) may be quite close each shot, but it may not be what you're aiming at...so you can learn to hold over your gun so your CPOI is the same as your POA. Like I said, not many people really need to worry about it, POI and POA are likely going to be close enough that your spread will cover you.

Another thing patterning helps you do is identify which ammo shoots best in your gun, i.e. which make gives the most regular spread, high dollar chokes are supposed to help regulate this as well. I've seen the same gun shoot spreads so lop-sided (about 85% of the shot fell to one side of the center, while the other 15% layed out for about twice the width as the rest of it gave) you'd want to throw it in the river with one make of ammo then turn around and throw the prettiest spreads you've ever seen with another make of ammo. Taking a shot on game with the first ammo might have ended up with a lot of misses or a lot of winged game, while the second make would have definitely been the best choice for hunting.

Basically patterning helps you know a little better where you shot is going. It's not an exact science, but it's close enough for government work...I pretty much only pattern to see how big my spread is at different ranges, and with one or two guns I do it extensively because I shoot reactive targets in competitions, if you don't have enough shot on the steel, they don't fall over, if you don't hit them at the top end, they don't fall over, sometimes if you don't hit them square and you twist them, they don't fall over, you either waste time and pick it up with another shot or you encur miss penalties (sometimes you incur the miss regardless of whether you neutralize the target in the end or not).