Re: Disintermediation
Hahaha, search engines. Cute. A normal website, appearing in the first page of results, if someone doesn't know the specifics of your product? (How would they know them, if you were entering the market fresh?) Not to mention, how much time, effort and money did it take to develop your sales website, and maintain it? I know from experience it does take some effort, especially dealing with things like sales problems/refunds etc...
The point is entry into the market is difficult for new applications. Doing it via the app store is easy - you create your app, and add it to the app store, providing detail in your blurb and categorising it properly.
Creating a properly SEO friendly website is a lot more work, getting it listed on search engines is not so easy - especially if its a competitive market.
The App store DOES handle some marketing. They sell handsets, those handsets are advertised as having access to the App store (which is prominent in their adverts). So much so, that they made $10bn in sales! If your app is popular within $random time period (this week, this month, whatever), it gets prominence in a top X list on there etc... (Which is advertising).
If you were starting fresh, the App store is a very attractive option.