The Importance of Mobile UI Design
The interface is your initial pitch to the user. You want to show your app holds value for them. A poor design immediately tells the user it has little value, no matter how useful it may actually be.
Think of it this way: Your interface has to cater to the most difficult person to reach, without alienating everyone else. It has to appeal to the broadest possible level of the population and show a clear value to nearly everyone who opens the app.