I was curious to the purpose a spirit beast poses and the benefits of the contract. I've been following the translation on Wuxiaworld for a while now so it's lost on me what purpose the beasts serve, unless their only purpose is to serve.
They are simply companions that add to your fighting/scouting/whatever strength. You can force them to be your servant or they can be your pet or friend. There is no special purpose except that they have some abilities you do not have such as void blinking.
None is a kind of a slave contract, but if they find someone powerful or kind, they can get all kinds of benefits like ning's beast, at the end of the day it depends on the type of owner.
They don't have any special purpose. It's basically either a slave/servant or a pet or ally/friend/companion or protector or something of the kind. The only benefit the contract serves is probably that you know they have no choice but to be loyal to you and that other people can't capture and contract them, because they're already contracted to you.