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.
Whatever the age of your pupils and regardless of the subject you teach, lesson plans can help you to ensure all course content is covered and help to make sure nothing important is missed out. A lesson plan is simply a detailed description of what materials are to be covered within an individual lesson.