So why build an app when you could more easily build a website that works on more screens?
There are plenty of reasons.
• Because you want to sell it.
• Because you want to make content accessible on a small screen, in the case of mobile apps.
• Because you want to leverage device capabilities: a camera, accelerometer, compass, GPS, iTunes library, in-app purchasing.
• Because you want to deliver content with reliability and immediacy, regardless of whether there is an internet connection.
• Because you want to create a unique experience that is designed for maximum impact on a particular device.
Otherwise, it makes sense to build a website.
Not an app.