Dorea already had a good reason and she gave it back to Han immediately after he finished asking.
"Ratte isn't just a coward. As of right now, Ratte is the only King that has a direct line to the Celestials and he is the only one that I can clearly say is capable of talking to a Celestial. Ratte is acting as a spy for them and the man that is directly in charge of me is the one acting as Ratte's handler. Do you understand Han? This much should be a good reason for him to live. That man is the only one capable of helping you with the Celestials."
Han clicked his tongue in distaste. He hated to say it but he could see the sense in what Dorea was saying. Ratte was a coward first and foremost, but as a spy for the Celestials, he can become a very valuable asset in the future.
As a Double Agent.