If you bet the flop and he doesn't have an A, he's likely to fold. If he has an A you just gave him money. If you check the flop, he may assume that you don't have an A, and his JJ,QQ, etc is still good and will bet the Turn. Yes, he may actually have an A, but you're f-ked either way on that... and you can't go folding KK everytime an A hits the board. This way, you maximize what you make when you actually are ahead, and potentially even minimize a little of what you lose when you're behind.