Super Standard?

[jrz1972]

Because you're giving villain the chance to show down his marginal/weak hands for free while value betting his good hands. Villain probably isn't going to bet the hand for you with bottom pair, but he's sure as hell betting an ace when you check to him. So if you b/f, you win a bet when you're good and lose a bet when you're not, while if you c/c, you win nothing when your hand is good and still lose a bet when it isn't.