A- the less you breast feed, the faster your milk will run out (the baby's suckling stimulates milk production)
B- breast milk is the best for the baby: it has components not found in formula milk, is lighter on the baby's stomach, and decreases the risk for all sorts of allergies (skin allergy, food allergy especially cow milk protein allergy etc.)
C- with your milk, the baby benefits from immunogloblulins (antibodies) from you that confer protection against infections
D- you lose pregnancy weight faster, plus it serves as natural contraception (prolactin, the hormone that stimulates milk production, helps in preventing pregnancy quickly after delivery)
E- it establishes a bond between you and your baby
F- it's much cheaper!
Of course it's OK to supplement with formula milk if the baby is not satisfied after being breast fed and your breast get dry - especially at night (formula milk is more dense and richer in calories so the baby gets full easier and sleeps faster), but I advise you not to make it a habit.