What we cover with iterative Fast Gradient Sign Method (iterative-FGSM) that we can't cover with simple Fast Gradient Sign Method (FGSM)

Question

FGSM and iterative-FGSM are two methods of generating adversarial samples. After visiting hundreds of sites and blogs to know that why in iterative-FGSM we perform multiple steps. And what we cover with multiple steps in (iterative-FGSM) that we can't cover in simple FGSM.

But all gives me this unsatisfied answer that in iterative-FGSM we perform FGSM multiple time. Please can anyone gives me satisfied answer to above bold question. Thanks