What's the idea of Prototyping?  

Basically, a low cost, low-risk approach for developing your product & evaluate whether it should be developed or not.  Instead of a theoretical plan on paper, you build a basic model of your App & see whether it is actually viable and practical.  

You can test the prototype with some users, close friends & your own company stakeholders with it. So, you can know whether you should invest more resources and energy into this product. Anybody who is an expert app developer should know the ideal App Development Process.

So, prototyping is an essential part of your product development for your Mobile App before you go into the real design & development steps.