Domain-Layer

Your ViewModel starts simple. It loads data from a repository and shows it on screen. Then product requirements arrive: filter tasks by priority, sort by due date, combine tasks with user data, validate input before saving. The ViewModel grows. It becomes 500 lines of mixed UI logic and business rules. Testing one rule means setting up the entire ViewModel. Use cases fix this. Each use case is one business operation. One class. One job. Easy to test. Easy to reuse across ViewModels. ...