For the 2013 installment of their Mobile Effects study Tomorrow Focus Media asked users about what is important to them when they decide on downloading/buying an app.
For the 519 users answering this question, usability and utility are the top factors, more important than price, reviews in app stores and friends’ opinions, underlining the importance of what a customers can see of your app in the app store.
The key is that users infer usability and utility from the scarce information visible on the app stores. Which, in turn, is why you need to be aware to make good use of these tiny pieces of screen estate.
(37signals’ Travis Jeffrey has some great examples for app screenshots.)