Understanding how Hub Sites work in Exchange 2013

Source: https://technet.microsoft.com/en-us/library/jj916681%28v=exchg.150%29.aspx

In your Exchange organization, you may want to force all message delivery through a specific Active Directory site. You can use the Shell to designate an Active Directory site as a hub site. When you do this, you cause additional overall overhead because more servers are involved in message delivery. For example, consider a message that's sent from Site A to Site E. If the least-cost routing path is Site A-Site B-Site C-Site D-Site E, and you designate Site C as a hub site, the message is relayed from Site A to Site C and then relayed from Site C to Site E.

You use the Set-AdSite cmdlet to specify an Active Directory site as a hub site. Whenever a hub site exists along the least-cost routing path for message delivery, the messages are queued and are processed by the Transport service on Mailbox servers in the hub site before they're relayed to their ultimate destination.