Having a course of action for your event is probably the most important part of event planning. Knowing in advance how many guests you want to invite, what sort of event style you want to have (cocktail, banquet, buffet, etc.), will it be casual or formal, will there be dancing or just background music, do you have to plan for anyone who needs handicap access, is there anyone on the guest list with special food requirements, will there be enough parking available, is the event location easily accessible to the majority of your attendees, and many more.
Having a good plan ahead of time means you have brainstormed and anticipated any needs you may have to take into account before you start making your booking plans. This way you avoid any unnecessary complications to your event by being organized from the get-go.