As they are being featured their promo art is shown to users before they get to the page for that particular app (see the screenshots for their new and old images - you can see it is part of a scrolling bar of promo images).

If they weren't being featured, the promo art would only be seen by users that clicked through to the app's page - users would only have seen the app's icon before getting the app's landing page.

So I think featured vs not featured makes a big difference to the importance of the promo art.