Luxon Scribes were vendors that allowed players to allocate Luxon Faction into either rewards for themselves or standing for their alliance. Luxon Scribes were needed for only two purposes. The first one is what's said above, which is to give faction to your alliance so you're closer to owning an outpost. The second reason was so that players could get closer to achieving their Friend of the Luxons title. In the Friday, June 15th 2007 update, they were removed from the game, and their services were transferred to the Luxon Scavengers.
The Luxon Scribe only allowed you to allocate Luxon Faction in loads of 5000 a go. You were not able to donate if you had less than 5000 Luxon Faction.