By allowing new functionality through new implementations.
The Open/Closed Principle states that software entities should be open for extension but closed for modification. Interfaces facilitate this by allowing new implementations to be added without altering existing code. For example, adding a new class 'DigitalWalletPayment' implementing 'PaymentMethod' extends functionality without changing the 'PaymentMethod' interface or existing implementations.
Additional Notes
How do interfaces support the Open/Closed Principle?