Can you give an example of a situation where using an interface provides obvious benefits?

Interfaces are useful when you need to define a common contract for different classes, allowing for polymorphism and easing the replacement of implementations.

Example scenario:

Suppose you have different classes for sending notifications — via email, SMS, and push notifications. All of them should have a Send() method.

public interface INotifier
{
    void Send(string message);
}

public class EmailNotifier : INotifier
{
    public void Send(string message) {
        // send email
    }
}

public class SmsNotifier : INotifier
{
    public void Send(string message) {
        // send SMS
    }
}

// Usage
void NotifyUser(INotifier notifier, string message) {
    notifier.Send(message);
}

This approach makes it easy to add new notification methods without changing the code that uses them, and simplifies testing through implementation substitution.