Mobile is now inseparable from everyone's life. Studies say that everyone spends more time with mobiles than with any other person or object, in the whole world.
If your apps are not mobile compatible, you lose market share.
Mobiles are handy and at the same time, they come with a unique set of challenges. There are too many brands and models, a whole lot of screen sizes and bandwidth differences.
This leads to a situation that you may have to test your app, with a variety of models.