While it may be true that experienced teachers do not write out elaborate lesson plans, nevertheless, for students learning to become teachers writing lesson plans can be very useful. The main benefit of writing writing lesson plans is to develop ggo habits of the mind. Habits that force the teacher to think about the important aspcets of the lesson and how to best teach and assess the lesson.
Lesson planning is a skill .
It is one thing to surf the Net and get lesson plans from other sites that you can use. It is quite another thing to have the skill to develop your own lesson plans. Having this skill is far more valuable than being able to use lesson plans developed by others. It takes practice to acquire this skill, and it won't happen overnight, but it is a skill that will help to define you as a teacher. Good lesson plans do not ensure students will learn what is intended, but they certainly contribute to it.