Planning is important because messed up planning, messes up the delivery of your writing.  You may have a great idea, but the best ideas can be doomed by poor delivery.

For example, here's a joke that's messed up:

The man says, "Great, and this will help my seasickness?" A man goes to 
the doctor. He says, "Doctor, I am going on a sea cruise for my honeymoon,
and my problem is, I get seasick!" Just before you get on the ship, eat 
all this stuff whole, without chewing." The doctor says, "No, but it'll 
look really pretty in the water." Doctor says "No problem, just
slice up one yellow pepper, one green pepper, and quarter 4 tomatoes.