Good UX is what separates successful apps from unsuccessful ones. Customers are won and lost every day because of good or bad user experience design. The most important thing to keep in mind when designing a mobile app is to make sure it is both useful and intuitive.
Obviously, if an app is not useful, it will have no practical value for the user, and no one will have any reason to use it. And even if the app is useful but requires a lot of effort, people won’t bother learning how to use it.