Why would you use the "Add Selected Subsystem" button when you can add a subsystem by double clicking on the home page?
With a single item selected the functionality is the same as double clicking an item in the list. However, the "Add Selected Subsystem" allows you to add multiple systems at the same time.
The list of plug-ins allows multiple items to be selected using the standard keyboard shortcuts for multiple list item selection.
Clicking the "Add Selected" button results in the System view opening with all the selected subsystems having been added: