Applications are not always brand new. We all use many applications that have been in use for years, perhaps decades. For mobile applications, refreshing the interface is a requirement, rather than a nice to have. If you are lucky, it will be delivered by updates to the underlying OS at little development cost (e.g. notifications in iOS 10). If unlucky, matching your interface to the esthetic of an OS may require you to redesign your app from the ground up. If done well, redesign can be more than refreshing for customers, it can be reengaging.