Consider the following scenario: you are about to implement directory synchronization for Office 365. You have multiple Active Directory sites across several, geographically dispersed, locations all over the world. Unsurprisingly, some of these locations have better connectivity than others and you might not want AAD Connect to connect to Domain Controllers in locations with a slow or high latency connection at the risk of slowing down the entire process.
When Azure AD Connect connects to a new forest, it uses DNS to locate domain controllers it needs to connect to. Without additional configuration, it is very difficult to control or know exactly which Domain Controllers AAD Connect will connect to. I believe that within the domain it is installed in, AAD Connect will try and connect to Domain Controllers within the same site first –but I’m still waiting on getting that confirmed. Even if that is true, that would not necessarily be the case for remote forests as there is no way for AAD Connect to know which site in the remote forest is closest.
Once AAD Connect is installed, you will find that it is relatively easy to define a (static) list of Domain Controllers that AAD Connect should connect to.