In Salesforce Marketing Cloud, the Contact Key serves as a unique identifier for each customer across multiple channels, allowing marketers to manage and recognize individual customers as a single unique profile regardless of how many channels they interact with. This is crucial for maintaining a unified view of the customer and ensuring that marketing efforts are coherent and consistent across different touchpoints.

The Contact Key enables a holistic approach to customer data management, supporting targeted and personalized marketing efforts across various channels, including email, mobile, social media, and more.

References: Salesforce Marketing Cloud documentation on Contact Management clearly outlines the role and importance of the Contact Key in managing customer profiles and ensuring a unified approach to cross-channel marketing.