A list of objectives can help you to gauge whether or not your students have achieved everything they should have once the lesson is over. Depending on the subject matter, objectives might be practical, for example, what can the student do at lesson completion. The objectives could also be knowledge based, for example, what does the student know after the lesson is complete.
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.