You are missing the concept of a protectorate. A country that is a protectorate of yours is under your protection. Countries/factions that invade your protectorate become at war with you. You are pledged to protect that country from outside invaders. There is no benefit other than ensuring that one of your enemies doesn't conquer them without taking you on, and you don't need to expend your resources in occupying them.

In terms of what they are doing for you, they will trade with you and not your enemies.