This is the extension to the things I explained in the previous reason. In a remote network, there can be different server roles and Active Directory integrated applications running. Similar to authentication requests, when users or computers try to use a service or application, it should process by the closest application server. It will improve the user experience and the reliability of the application or the services. When it's via slow link, it does have lots of dependencies and performance-related limitations. In order to do this, there should be a system, which can process these service requests and point users to the closest servers.

The answer for all the aforementioned requirements are the Active Directory sites and related components. These allow us to represent the physical network topology within the Active Directory infrastructure. Then, Active Directory will be aware where its components are located and how it's connected with each other. Based on this data, Active Directory will allow to control the replication over slow links and point the authentication and service requests to the closest servers.

Let's look into Active Directory site and its related components.