Before we dive this huge question, lets start by understanding the word “Usability and its history. Debates on usability have been around for quite a long time, since the beginning of computing era. But the real turning point came when personal computers came into fray and software was developed for mainstream users. This is was the time when the necessity for USABILITY of applications gained momentum. And with the age of mobile apps, usability became the most important entity in apps production.
To answer the question above, we need to understand why usability is important. Usability is the moment of truth” with the customer. As per statistics users approximately takes 50 milliseconds on an average to shape their opinion on any online product. And the duration on the application remaining installed in the device is based on the usability. In this competitive age, where we have thousands of apps for a same purpose, only usability decides the fate of an app. Usability can be substituted with a term user friendliness” which again can be simplified as the ease of efficiency with which the user can interact with an app and get his need fulfilled.Mobile apps are mostly utilized by users on the go. And users go to mobile apps mostly when they have time constraints. Features typical for user friendly apps include size, speed, data usage, graphics, visuals, etc., So psychologically customers prefer apps, which are simple to understand and easy to use.
Not always a simple app with minimalistic design is needed. For example, apps for children might need bright colors with images and cartoons. That defines usability, user interface developed based on the user.
So the verdict, - Focus on the user and all else will follow. This is not from me, this stands number one, in the list ten things Google as a company, believes in. And I think that this sums up the topic and gives us a valid guideline.