Laurent Bossavit asks a great question, "What is the purpose of planning?". He then gives examples of two of the most common purposes: to facilitate completing the project by some significant date or event, or to facilitate predicting the budget. I like Laurent's question, and encourage you to ask it before (or during, or after) you engage in any significant planning.
"What is the purpose of planning?" is a form of The Value Question. However you answer the question, whatever your purpose for planning, your answer expresses a belief that planning will achieve that purpose. I want to explore that belief by asking another question: How will planning achieve that purpose? How, for example, will planning help you to complete the project by a significant date? How will planning help you to predict the budget?
Planning helps you do those things by making explicit your thoughts about what resources you will apply and what will happen as a result. Planning exposes your commitments, expectations, and assumptions, and allows you and other people to explore them, to test them, to assess your confidence in them.
Whether your expressed purpose for planning is to help you meet a committed date, or to predict your budget, or some other purpose, planning always has this intermediate purpose: to communicate our present commitments, expectations, and assumptions about the project.
One implication of this purpose is that though planning seems to be about the future, it is, ultimately, entirely about the present, about what we are expecting, assuming, and committing to right now.
Experiment: The next time you plan, make this intermediate purpose explicit: to communicate and examine our present commitments, expectations, and assumptions about the project.
Experiment: If planning is entirely about the present, about the commitments, expectations, and assumptions you hold right now, what information becomes more important to your planning? What information becomes less important?
Experiment: If planning is entirely about the present, how does that affect the way you treat the plan as the project progresses?