The Display node is an important node; it is used by the export and views to display your scene, primarily the Camera and Timeline views. By default, a scene's node system always has a Display node which can be used to show your entire scene, a single node or a group of nodes through a Composite or Group node. You might use it to see one single character as you animate it, debug your node system as you create complex effects or export your complete scene or a portion of it.
Harmony Advanced doesn't allow you to add Display nodes. The display selection feature is only available for the possible scenario that you need to use a specific Display node created by a Harmony Premium user.