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.
Are you planning on teaching a class? If so, lesson plans provide you with a variety of different advantages. Any experienced teachers will tell you that having a plan does wonders for smoothing out the most chaotic of classes. Others, however, will tell you that sticking too closely to the plan will make things needlessly complicated and that flexibility is the key to running a sound classroom. In truth, both sides have merit; the best way to run a classroom, particularly one filled with younger, excitable students, is to maintain a perfect balance of order and creativity.