The function of a brake booster is to prevent the brake bosses from moving apart under heavy braking. A good brake booster considerably improves the feel and the strength of the brakes – the force of your finger is converted to braking instead of flex of your fork or frame. It is especially important if you have flexible frame/fork/brake bosses. Therefore, the main characteristic of a booster is its stiffness. Not strength, but stiffness. Or actually stiffness-to-weight, because you wouldn’t like to carry half kilo of steel on your bike. Link.