Depending on the type of marketing you do for your app, being discoverable in the App Store can have a big impact on sales.
One of my apps (Romote) is a WiFi remote for Roku 2 streaming devices. I wrote this app mostly for my family’s use and only put it in the App Store since I already had an iOS developer account. Much to my surprise, the app has been a consistent seller at $0.99 (with no outside promotion) even against free competition (there are a few features that differentiate it from its competitors like correct handling of dication in search fields and an iPad optimized layout). While the volume of the sales were nothing special, they were consistent - at least until they disappeared entirely.
Around the end of last year (2012) the sales regularly dropped to zero per day and stayed like that for multiple days at a time. At the time I just assumed that people were no longer interested in paying $0.99 for an app when there were free alternatives. Then while searching for something in the App Store I happened to try a search for “roku remote” and was quite surprised when Romote did not appear at all in the results. Searching for the app name itself returned the app in approximately the 150th position, behind such apps as “Fart Studio - Revolutionary New Farting Surface!” and “NASA Earth as Art”, as well as several of its competitors.
I contacted Apple developer support about the problem and received a predictably vague response about many factors affecting an app’s search ranking. While I understand the need for circumspection in these matters to prevent gaming and abuse I was frustrated that my app was not appearing for what I considered very relevant searches. Whether as a result of this interaction or just via an update to the search ranking algorithm, Romote soon began appearing in results again and the graph above clearly shows the impact on sales.
I think this illustrates a problem for apps that aren’t receiving promotion outside the App Store (via reviews, blogs, etc.). The discoverability of the app in the store will greatly impact its sales - this is even more true with the iOS 6 App Store redesign that shows fewer search results at a time. The part that is a problem is the fact that you have almost no recourse when this occurs. Adjusting app names and keywords in an attempt to appear in results for truly relevant searches is a Sisyphean task. This is analogous to Google controlling the majority of search traffic on the web (and with the same games being played by unscrupulous players) - if your product/site isn’t returned in Google searches your traffic will suffer.
I don’t think that there is a real solution to this problem (other than outside promotion) since Apple controls the App Store - that’s the price of admission for making your app available to the millions of iOS users out there. But it is something to keep an eye on and be aware of, especially for smaller developers.