make teachers aware of the aims and language contents of the lesson, so as to plan the activities and choose the techniques accordingly
helps teachers distinguish the various stages of a lesson and see the relationship between them so that the activities of different difficulty levels can be arranged properly and the lesson can move smoothly from one stage to another.
gives teachers the opportunity to anticipate potential problems so that they can be prepared,
give teachers, esp.novice ones, confidence in class
rasises teacher's awarness of the teaching aids needed
planning is a good practice and a sign of professionalism
to enable the teacher to improve class timing
lesson plans are also an aid to continuing development