Interfaces separate the definition of capabilities from their implementation.
Loose coupling means components in a system are independent and changes in one element have minimal impact on others. By using interfaces, you can define a contract and change implementations without affecting the clients that depend on the interface. For example, switching from 'SQLPaymentProcessor' to 'NoSQLPaymentProcessor' in an e-commerce system can be done by simply implementing the same interface, ensuring minimal changes in the rest of the code.