Why does every forest need to have at least one Global Catalog (GC) server?

Every forest requires at least one Global Catalog (GC) server primarily to enhance the efficiency of directory searches across the entire forest. The Global Catalog holds a partial, yet comprehensive snapshot of all objects within the Active Directory environment, including essential attributes that may be needed for common queries. When a search is conducted for a directory object, the Global Catalog significantly speeds up this process by allowing the system to quickly locate the necessary information without having to query each domain individually. This becomes especially evident in multi-domain environments where users often need to find information across different domains within the forest.

In contrast, while external access, centralized user management, and domain replication are important aspects of Active Directory architecture, they do not specifically pertain to the function of a Global Catalog server. External access relates more to how users outside the domain can authenticate or connect, centralized user management is focused on Streamlining administrative tasks across the network, and domain replication deals with the synchronization of data across domain controllers rather than directly impacting search speed.