There are two reasons to upgrade the database. The one is we received a new InstallDeveloper, so we need to upgrade system resources (tables, agents, forms etc). The second is that we have created a new InstallServer and we need to upgrade the local resources (tables, agents, forms etc) in the customer's place. The local upgrade contains also a total upgrade of the database including the system resources.
For the System Database Upgrade
For the Local Database Upgrade