On one side it allows the team to get a better understanding of the customer's business processes and get closer to the specifics of his company, on the other hand, it helps the customer to test the viability of the idea or startup and save a decent amount of time and budget.
As for the IOS prototyping and the Android app prototyping, we involve professional UX/UI designer. The expert builds his work according to central principles of usability, iOS and Android (Material design) UI-guides (GUI - Graphical User Interface) and requirements of the business.