People seem divided on this. On one hand, it can be used for marketing. It is, after all, one of the very first things a user will see about your app. When they search for it or see it on the App Store, your icon will be there to try to sell the app.

On the other hand, once a person becomes a customer and has your app on their home screen, the purpose suddenly becomes very different. Users should be able to recognize your app icon when they’re looking for it.

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I believe good app icons are most valuable for your customers, your everyday users, not first-time shoppers.

— @Mantia