A good bra is one that's comfortable, that fits you, and that you like. Honestly? That's up to you. If you don't want to wear one, you can choose not to. If you choose to wear one, it can be because you found one you think is cute or fun. It can be to help support your boobs when you run around. And if they're bigger, it can help to give them support to help with your posture and your back because the bigger they are, the heavier they are, and that can take a toll on you.