Because babies breast feed for months, ideally anyway. The hospital breast milk is really important for premature babies (cuts down hospital stays), and is really healthy for newborns regardless. But babies ideally breast feed for at least 6 months (The pediatric society might say much longer than that, easily googleable and telling of my own laziness on searching).

Assuming there are absolutely no medical or circumstantial reasons to use formula (such as the complication with work in my original reply and public acceptance of breast feeding in public), there aren't many reasons. You know, except being able to be away from your child for any length of time in the first months. But this simply isn't reality, nor is breast milk always enough (my mother simply couldn't produce enough milk for my little brother to stay healthy, and his asthma was already a concern). Some women die in childbirth as well. Not to mention psychological concerns (I refuse to have a child. Pregnancy and breast feeding are the stuff of nightmares for me). We need the option there just to make sure all babies have nutrition.

And the more folks push, the more acceptable it is to shame non-breastfeeding mothers. That really freaking bothers me.