Why is it necessary to separate the display and business logic in an application?

Separation of presentation (UI) and business logic is critically important for several reasons:

1. Improved testability: Business logic that is independent of the Android framework can be easily tested with unit tests on JVM, which is much faster and simpler. UI logic requires instrumental tests.
2. Maintainability and modifiability: Changes in the UI do not affect the business logic and vice versa. This simplifies support and adding new features, as changes in one component minimally affect others.
3. Reusability: Business logic can be used in different parts of the application or even in different applications (for example, on other platforms), if it is not tied to a specific UI.
4. Parallel development: Different teams or developers can work on UI and business logic simultaneously.
5. Data handling convenience: Business logic operates on clean data, transforming it for display, which makes data management more predictable.
6. Compliance with SOLID principles: Separation helps adhere to the Single Responsibility Principle (SRP).
7. Platform adaptability: Changes in the activity/fragment lifecycle or other Android components have less impact on business logic.

Architectural patterns such as MVP, MVVM, MVI, Clean Architecture help achieve this separation. In these, the UI layer (Activity, Fragment) is responsible only for displaying data and forwarding user input events, while business logic resides in other layers (Presenter, ViewModel, Use Cases).