Quote Originally Posted by godsakes View Post
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.
Its not only for quickly controlling an area blitzing and good troops. Some regions you just have to have one or you will never hold them unless you want to keep your faction leader in the town for 10 years :/