A strong plan is invaluable in generating outcomes that match the desired ones discovered in the initial meetings and design phases of any project, but especially in larger and more complex projects where parts of the work will be delegated to team members that will be responsible for parts of the whole.
When I was younger, and part of one of our Canadian cadet corps promoted to command of a section, I had a commanding officer that would tell us what we had to do, when it had to be done by and what things we were not allowed to do. On each project I am engaged in I look for these guide rails.