Bro stole her skills, there wasn't any need to actually win I don't think there was any po...

I mean it's not like he knew what the reward would be. He did have a reason to participate though. If he didn't, he could have lost favor with the Constellations or they could have pulled something to make it happen anyways and probably make it be dangerous out of annoyance. There was no reson to not participate. He also had to know that there was going to be some form of reward for entertaining the Constellations request, so he only stood to gain something from winning whatever it may be. It was also a good way for him to test his abilities so why not try his best and see how well he could do against her?