Once you have finished your plans, you will find that your class runs much smoother than it would without it. For example, your students will enjoy the luxury of the daily routine and the security of knowing that their teacher will know exactly what the plan is on a day to day basis. Should a delay occur, such as a classroom discussion tangent or a school cancellation, you will nevertheless have enough time to reschedule various events. Writing plans are also useful in that if you plan to take any field trips, you will be able to prepare for these events ahead of time.
Ideally, grade lesson plans should be orderly and sensible, but with enough room to include spontaneity and creativity. Anyone who has had to sit through a lesson plan that was rigid and tight, with absolutely no deviation from the tasks at hand, will know that this is a thoroughly unpleasant way to learn. Letting your students breathe a little is important in helping them learn and remember.