Each lesson the teacher gives is a unique experience for students. They experience each lesson only once. Thus, the teacher should think a great deal of each class and prepare things sufficiently for it.
This is the most important reason why a scenario of instruction should be made for each lesson.
Indeed, in the process of making a scenario, teacher can find what is necessary, what should be remarked, what lacks in him, etc. for doing the class. To Make a scenario is to rehearse a class.
There is also a case where a teacher shows his/her scenario of instruction to others in order to get their comment on his/her way of instruction.
If a teacher get a chance to demonstrate his class to others, he/she should prepare scenarios of instruction.