Like baseball, ground rules make it possible to hit home runs. Without ground rules, small issues become major time wasters and important matters do not receive the attention they deserve.
Ground rules discourage someone from monopolizing the meeting with personal concerns.
An agenda is the meeting map. Everyone knows where they are going and what the final destination will be. Without an agenda, any topic is fair game. While it is conceivable that every topic might be of interest, the ability to act on each is limited due to lack of preparation. An agenda is critical for staying on course.