I used to coupon, not extreme, but like regular coupon.
I didn't understand it until I did some research, but the reason that they buy so many of a particular item is because sale prices go through phases. More sophisticated couponers watch sale prices and keep a track of "rock bottom" prices, so when something you buy gets to or below your rock bottom price (which for many means with the use of a coupon they pay little to no money) it's time to stock up.
You buy a lot because it will be a while before whatever item will get to your rock bottom price again. I don't think they tell you that on the show, but that is why people keep a stash or stockpile. These people aren't buying things to fill their pantry every week, they buy when the things are little to no cost and keep a good amount to last. So they may go to the grocery store and only get 10 things, but they'll buy a sh*tload of those 10 things to last until it gets to rock bottom sale price again. And this is repeated basically weekly.
Don't be loyal to one store. And sign up for store cards with your correct address. Many of them will mail you store coupons directly which can be used on top of manufacturer coupons (stacking) or total purchase coupons.
Another thing-some stores won't give you overage (I think wal-mart may be one) so like mentioned you have to be very aware of store coupon policies. One's that do give you overage-that can be used to buy (fresh) meat, seafood, veggies and fruits. Almost anything else in the store you can get a coupon for.
I've gotten many free things, and some for low cost. I've gotten free cake mix, lotion, hair conditioner, milk, and other crap. And many, many things for low cost-cereal is a big one, yogurt, juice, makeup, and again many many other things.