Contributed By: Cliff Hobbs [MVP ConfigMgr]
THE biggest reason for having multiple Site Servers in your hierarchy is network bandwidth. Very few (if any) companies have unlimited bandwidth between locations. If you install a ConfigMgr Site the remote end of a WAN link you can control when and how much of the link is used, plus any traffic sent between Sites is compressed. If you don't install a remote Site Server then all of the traffic is sent to the Clients uncompressed, at an unpreditable time, and the same traffic is sent across the WAN for each Client.

Another reason for additional sites could be your requirements are different between locations. For example at HQ you want to run Inventory every 7 days whereas at a different location they want to run it once a month. As Inventory is a Site-wide setting (and there are many others), to meet this requirements you'd need separate Sites with each configured accordingly.

You also have different locations in different countries with the requirement to support different languages. For example you might have Administrators in Portugal who need the ConfigMgr Admin Console to be in Portuguese whereas in London you want your Administrators to see an English console.

For further details sees the following:

Configuration Manager Multiple Site Planning and Deployment:
http://technet.microsoft.com/en-us/library/bb693854.aspx

Plan the Configuration Manager Site Hierarchy:
http://technet.microsoft.com/en-us/library/bb680436.aspx

Planning and Deploying Your Multilingual Site Hierarchy:
http://technet.microsoft.com/en-us/library/bb680663.aspx