Sure.
1. I am the one with the big stick. Their navy is part of my navy. Having large fleets is fun but, i tend to stay under my fleet cap so, my vassals effectively make up roughly 10-75% of my navy, depending on how many vassals I have.
2. Free pops/Free real estate. Just look at all that land I can use for more vassals. I tend to annex/integrate my vassals only to make even smaller vassals out of them, of course, I steal about 25-50% of their pops before releasing.
3. I hate you, now die. I get bored from time to time. "I have this colossus, now what? Oh yeah. Let's set him free, then wage unholy war. Galactic community can't hurt me." Why would I blow my own planets up when I clearly got people dying to be my subjects?
4. TAXES!!! Personally, I hate/love tributaries, problem is they don't join my wars, so I break them apart into smaller vassals and designate some to be tributaries. Once Overlord comes out, this is going to be fun and heavily tax the crap out of the AI.
5. My ethics demand all bow before my great king. Militarist, Authoritarian, and Xenophobe. My Authority is clearly Imperial (Mainly cause I'm lazy and I use a lot of Edicts). Long live King Charles "The Greatest Plan" XVIII
6. I'm lazy and don't wanna manage that big of a empire myself. I can do it, just I don't wanna. Besides, they're all loyal to the crown, right?
[Insert funny revolt skit]