Always create a web site to promote your app, aka app landing page.
Yes, websites are primarily for desktop users. But you’re already adopting the “mobile first” strategy by creating an app in the first place – a website is just a way to draw more attention to it. You need a place to give all-important bloggers, journalists, and other online influencers a place to find you. An app can’t go viral by itself, but a web URL can – it is sharable through blogs, social media, and anywhere else on the web. This also gives users another way to find and communicate with you. Finally, as you add more channels to your marketing mix – a website allows you to have a simple, one-click call to action. This is a much likelier conversion than say, tweeting about your app, hope people immediately pick up their phone, enter the app marketplace, find for your app and (finally!) download it. Ultimately, the user will have to take these steps to download the app either way, but which would you consider more compelling: a tweet with a 140 character limit, or an entire website that intrigues and excites the user about specific benefits of an app?