Why do we lesson plan?

(Special thanks to Thomas Sauer and Pam Wesely for their interesting conversation that inspired this post.)

While wandering the wonders of Twitter, I came upon this conversation:

Pam's last tweet says: "usually used shorthand for day-to-day practice."

This lead to me an important question: why do we lesson plan in the first place? A good friend (Stephen Vrla at http://housingincluded.blogspot.com/) said that it helps us make sure that we as teachers fully understand the material before we share it with the students and that written planning helps us organize what we know in the most effective, student-focused way possible.

And Mr. Sauer is completely correct: through lesson planning is time consuming and requires a good bit of mental energy. Plus, in my experience, it is somewhat boring, especially when you're doing it by yourself.

A few initial thoughts on why we lesson plan, beyond Stephen's original idea:
1. It gives us something to look back to, reflect upon and revise later.
2. It helps provide clear evidence when teachers need to explain themselves to others.
3. It provides a sense of stability and safety for the teachers (always having something to fall back on).

But is lesson-planning student-centered? My middle and high schoolers are completely capable of making a game-plan by themselves. Furthermore, it makes class theirs and not mine. It allows them to be more excited about the material because they own it.

I'll be honest. Some times I worry that when I let the students plan a lesson, that they won't get as much out of it. It's hard for non-heritage Spanish speakers to plan how to learn Spanish. How can they plan for things that they do not even know exist?

Then I remind myself that
a. I need to let go: this is about their learning not about passing a test. I came to a private school exactly so that they wouldn't have a test to pass.
b. I am still there to provide guidance. My Spanish abilities and the dictionaries and the internet does not magically disappear when the students design the class.
c. This is not "lazy" teaching. It's just different.
d. students can gain so many more skills, such as time management and collaboration, if they do the leg-work by themselves. If I teach the lesson, I do all the work. If they plan and execute the project, then they do all the work. Work (one hopes) = learning.

I've had some success with the student planning method (see the post on extinguishers below for an example) although it can lead to an uncomfortable level of instability and unpredictability during class.

In the end, I do not know which is better, teacher lesson planning or student lesson planning. I lean towards the students doing the planning.

Thus I pose two questions: why do we lesson plan? and Which is better for the students, that they plan or that we plan?