Why was Vercingetorix put in prison for 5 years?

Just curious - when Julius Cesar defeated the Gauls, why did they put Vercingetorix in prison for 5 years before executing him? Did they need information from him or what? Was it normal for Romans to put people in prison for a few years before executing them? Just kinda seems like a long time to wait to execute someone, especially when the Romans weren't shy about asinations.