Why buy the cow if they're getting the milk for free?

Could be the fridge isn't big enough to conveniently hold whole gallons. Or maybe they walk/bike to the grocery and the half gallons are more convenient to carry back from the store. Or maybe some member(s) of the household can't lift whole gallons of milk. Or maybe they're like my family which will drain a gallon in two days then go 2 weeks of barely touching the milk before suddenly devouring milk again. Or maybe 2 members of the house independently decided to pick up a fresh half gallon of milk on the way home on the same day and now they have a gallon of unopened milk