When it comes to why a user decides to download an app, the app icon is 13th on the list, according to this study of app users. I was surprised by how far down the list it was. It's good to keep things in perspective, though.
Another piece of info is that over 36% of app users either rarely or never look on the App Store or Google Play Store. Interesting. What can that tell you about app discovery? Be sure to read the other findings so you can prioritize what you spend your time and money on.
Consumer Study: App Discovery, Downloading, and Purchasing by Ben Bajarin