2 main reasons:

quickly control an area - client rulers make a huge difference in keeping a province happy, extremely handy when you're in a situation where you need to move your army on and are unable to quickly reinforce the area. Also when your ruler dies of old age you can easily replace them and not mess around shipping family members across the world

types 4 govts get the +1 experience bonus, when combined with blacksmith/temple bonuses (and assuming the region is capable of producing worthwhile troops) they can become some of your best troop production centres.