When an identity tries to authenticate, the request should be processed by a domain controller. If all the domain controllers are located in one geographical location, it doesn't matter which available domain controller processes the request. But if it's between remote networks, the time it takes to process the request will depend on the available link bandwidth and the number of hops it needs to travel through.
As an example, let's assume Rebeladmin Corp. do have an Active Directory infrastructure and it is stretched across two offices in London and Seattle. It has domain controllers located in both the locations. If a user logs in to a PC in the London office, it doesn't make sense to process the authentication request by a domain controller in the Seattle office. This is because the request needs to pass through a few network hops and a slow link. If large numbers of requests are processed, majority of the slow link bandwidth will be used by these requests. Ideally, it should process by the closest domain controller, which is located at the same location. Then, there are no additional hops to pass and no bandwidth limitations. Also, it will not depend on the status of the link between two locations in order to process the requests. Therefore, Active Directory should force identities located in remote networks to authenticate via its closest domain controllers.