If you do not have a plan when things go wrong, what do you do? You are trusting yourself to make a reasoned choice quickly amongst chaos and emotion. Planning is not some magical thing where you have to tirelessly plan out every scenario or every detail - as people who hate planning might think. Nor is thinking that the plan will need to change anyway so why bother doing it in the first place.
It is the planning, rather than the plan itself, that is beneficial. It is the thinking time ahead of chaos that allows better decisions when the plan is knocked off track, even if you have not formally considered the event or circumstances that end up impacting the plan.
Similarly, it is much easier to assess what is going on in life through a mental model. We have mental models whether we consciously choose them or not, so why wouldn't you want to be conscious of them! To be more aware of yourself, you need to be aware of the mental models you are using.
A plan is very to mental model, it helps make sense of what is going on around you, and the consequences, very quickly. It provides structure now, to any further decision. With no plan to start with, things will quickly descend into scattered panic and disorder at the first sign of trouble.
This is what leaders do. They must sell the vision of the future, gain buy-in and then get on track through a plan.
Meetings are no different. They will descend into mindless chatter and chaos without knowing the vision of the meeting - the purpose. There must also be a semblance of structure - an agenda. It does not need to be detailed, but it does need to be there to protect the purpose of the meeting.
Things will go off track but deciding how to get back on track is much easier now you have done the groundwork.