If the formula is there, it will be taunting you that first night home when you still aren't quite sure you can do it. Formula gives a lot of women peace off mind. You know exactly how much you're baby eats and exactly when he eats it, it tells you when to increase his feedings, etc. Breastfeeding can be kind of scary for the first few months. All of the sudden you go from feeding baby ten times a day for twenty five minutes to twelve times a day for forty minutes. You just have to remember that cluster feeding doesn't mean hee isn't getting enough. It means he is prepping your body to make more so he can have a growth spurt.
I wouldn't keep formula at home if you're breastfeeding. Too much temptation.
No you dont have to buy formula. Breast milk is all he needs :)