My understanding is that to display a rewarded ad you are required to ask user if they would like to see it. Interstitial rewarded ads can be shown automatically without a prompt during natural app transitions.
Edit: Apparently, you still need to give user an option to opt-out:
With interstitial ad you only get paid if people click on it.
With rewarded and interstitial rewarded you get a small amount of money when the ad is played to the end.