Professionals planners help to create the vision for the event. They lead the team in researching, designing and developing the event to make sure it fulfills this vision.
Planners are interested in the big picture and the small details.
They don’t just do this at the start of the project, they do it ongoingly throughout the whole life-cycle of the event and make course corrections if the vision is not being fulfilled.
Creating a plan should happen before anything else, even booking the venue. After all, how do you know what kind of venue to book until you have a vision for what the event will be?