Courtiers and traders have severe limitations that limit their gameplay potential. Traders can only have mercenary troops and courtiers police troops, for example. So anyone else can have their troops as police or mercenaries whenever they want to, but these guys are stuck with nothing but that.
They were way more common when people had two characters per realm, one of them being more of a secondary character that could do more support actions without the player completely missing out on all the major events.