How does properly separating business logic from a specific state management library, such as Provider or BLoC, help make that business logic genuinely reusable and considerably easier to test, regardless of which particular library you might actually choose to use?

Keeping your core actual business logic, such as use cases and repositories, fully independent of any specific chosen state management library means that logic can genuinely be thoroughly unit tested completely on its own, without needing to also set up or involve any actual specific Flutter widget or state management framework at all, and it also means your specific state management choice becomes considerably more of a genuinely replaceable implementation detail, since your core actual business logic itself does not depend on or even know anything about that particular chosen library, making a future potential migration between different state management approaches, such as from Provider to Riverpod, considerably less disruptive and risky overall.

// Business logic has no dependency on Provider, BLoC, or any specific UI framework
class OrderService {
  Future<Order> placeOrder(Cart cart) { /* pure business logic */ }
}