The base architecture of iOS is MVC (Model View Controller) which leads into known as Massive View Controller, where the View Controllers end up doing so many logic. It is easy to mix UI code and business logic together which is the wrong way to test the logic.
VIPER is one of a modern architecture made for this issue. It is based by Uncle Bob's Clean Architecture. It use the Separation of Concern principle which make the code cleaner and easy to maintenance.