User interface design is about the nuts and bolts of a mobile app. User interface design is as much about functionality, as about aesthetics - the look and feel of a mobile app. Both functionality and aesthetics are core attributes of a mobile app development process.
For instance, at Litmus Branding, we understand that if you are a news app, you have to constantly churn out engaging, dated content. That's the functionality part of it. But, along with it you must also look newsy and buzzing with real time content updates.
Likewise, if you’re a reservation app, you have to be sharp at offering hot local restaurants deals, ahead of competition; and the display of the options must be appetizing too, otherwise why would a customer hit the 'buy' button?
Understanding user preferences and improving user interface of your native app is thus critical to the success of all app branding/marketing/monetization efforts.