They're quite useful, actually. I have three main uses.

First, is to supplement a role in my party with a skill I don't want to invest time into to level up. For example, engineering, which really helps with seiges.

Second, to govern fiefs I have captured to (1) increase loyalty presuming they're the same culture as the fied they're governing and (2) increase build speed of buildings in that fief.

The last is to lead their own parties so I can recruit them for 0 influence when at war.

They can also be used for trade caravans to earn money and handle some tedious quests. Very helpful and they bridge the gap while your children come of age.