An account must be set that will be used to run the ITP/Server processes.
If ITP/Server is installed on one machine this account can be the Local System account because the ini file is on the same machine as the ITP/Document Processors (The crux is that the ITPWORK folder is on the install machine). The Local System account has full administrative (= unrestricted) rights on the server it is running on, but has no rights on other computers in the domain. This might be a reason to use another account because access to applications that are installed on a central network server might not be accessible. Another reason is that the Local System account has access to local resources, which should remain inaccessible.
If ITP/Server is installed on more than one machine the local system account will not work at all, because the ITP/Document Processors need access to the ini files somewhere on the network.