I have been looking at auction theory and in the book Auction Theory by Krishna, there is one (seemingly simple) inequality that I just cannot follow.

Context: given a private valuation $x$, the optimal bidding strategy has been found $\beta(x)$. Now, the author wants to show that behaving and bidding as if you were of type $z$, $\beta(z)$ does not increase profits.
Then, calculating the difference between the profit in the optimum and the profit if you would behave as if you were of type $z$ leads to the following inequality.
$G(x)$ being a probability distriubtion:
$$\pi(\beta(x),x) - \pi( \beta(z),x) = G(z)(z-x) - \int\_x^zG(y)dy \geq 0$$

The profit functions were calculated from a first-price auction in case it helps anyone.
My question is why the inequality holds. Why is $G(z)(z-x) - \int\_x^zG(y)dy$ larger than 0?

I hope you can help me :)