Currently, there is one field, "Mail server", containing the server handling incoming emails for a user. If I'm not mistaken, this field is also used to create the initial Online location for the user (field "Home/mail server"). If I change the field "Mail server" in the person document, another server will receive the emails but the location is not modified.
We don't want to keep the field "Home/mail server" in the Location document in sync with the field "Mail server" in the Person document because we would loose flexibility.
But with a second field in the Person document, "Home server", usually empty (which means, "Use 'Mail server' field value"), we will have a possibility to control the field "Home/mail server" for a specific user. And we will do this without accessing the user's computer as we have to do currently without this functionality.