What is difference between Promo Graphics and Feature Graphics in Play Store

update one point: The promo graphic is not needed anymore, except for users still using earlier than Android 4.0

https://support.google.com/googleplay/android-developer/answer/1078870?hl=en

The promo graphic is used for promotions on older versions of the Android OS (earlier than 4.0). This image is not required to submit an update for your Store Listing.

Promo graphics is promotional graphics, popular featured apps with high ratings get to display the promotional graphic.

Selected applications which are trending or most downloaded or chosen by editor in their respective categories get their promo graphics visible at the top. This is not guaranteed, but uploading promotional graphics is something which is recommended all the time, as you never know when your app tops the list.

NOTE: The promo text has turned into a short description and is now shown on the main info page, before the user presses it to view the full description. Not all of the promotional assets are required, but all are recommended so that your app can participate fully in the Android™ Market.

At the same time, the Feature Graphic is used for promotions on Google Play. Graphic is required to save and publish your Store Listing, it is required in order to be featured on Google Play.

To restrict the promotion of your app across Google-owned properties, you can check the "Marketing opt-out" tick box on your app's Pricing & Distribution page.