Flying wraiths lack firepower, so they provide no real air support. Just fast transportation/communication.
If not for Gandalf using his own metaphysical powers to give hope to the soldiers, I imagine the gondorians would have become inactive with shock due to terror and dread in the airwaves.
And the communication and intelligence that flying wraiths would allow is no small asset. Part of the advantage of a walled city is a view of the enemy approach while hiding your own formation. Good areal surveillance eliminates that advantage for the defending side.
The only canonically named Ringwraith was Khamul, the Witch-king's number two. (Roleplaying game company Iron Crown Enterprises once put out some nice but totally unofficial names for all the Nazgul.)