If your baby uses disposables, you'll buy thousands on the way to the Golden Throne. So, saving a few pennies per diaper adds up to a couple hundred bucks.
Diapers are, after all, poop bags. So why are you buying extra-expensive "supreme" and "ultra" poop bags? For most purposes, the store brands do a fine job of absorption and containment. As long as you change them often (and quickly after a poop), your baby's butt will never know the difference. But your wallet will.
(After you read the comments, the discussion continues over here. — Ed)