In the Properties of the TwinCATCom driver, the UserName and Password properties are used by the driver to insert routes in the remote AMS routing table.
For instance, if you have a CX8090 which is WinCE based and you did not change its password, in the AdvancedHMI driver, you would put Administrator for Username and leave Password blank. When you start your HMI application, the driver will automatically add the routing table entries for you.
The advantage to this is that you can move the HMI to another PC without having to configure anything on the CX controller.