Writing Plan? Really Necessary?

Have you ever seen that comic strip where Calvin said that he was waiting for the mood to write, then inspiration to write, then finally, he would resort to last-minute panic to start writing?:

This is definitely NOT my writing plan! What is my writing plan you ask? Well, I like short things to write, like a paragraph or two, due the day after it was assigned so that I can not procrastinate. That way, last minute panic is hard for me to have.
I do agree with Calvin when he said that you have to be in the right mood to write. For instance, if I really feel like doing math (which is not very often) I might give myself math problems all over the page instead of writing, or if I am in the mood to draw, then I might make very detailed illustrations of what I want to write instead of writing it. Sometimes, all I have to do to write is to be awake!
Often, when I write, I have to look for inspiration. How do I do this, you ask? Well, I sometimes go to google images and type 'inspiration', but that hardly ever works. Sometimes, I look at a random book and find the first word I see on a random page, and, as long is it is a noun, I will write about it. Unlike Calvin, I tend to look for inspiration, not wait for it.
But do I even need a writing plan? Was all of that inspiration-finding, mood-finding due date making necessary? I don't really know and I am not sure that I want to try writing without it because of the impending disaster that may come without a writing plan. Calvin didn't really have a writing plan and it ended in last minute panic.
Last minute panic is by far the worst way to write. The worst way out of the worst ways to write. When I am panicking, I tend to stay very awake like I was just injected with very strong coffee, and I get jittery. I personally can NOT write while jittery. I know someone, who, when panicked, falls asleep. I can not write while asleep, can you? I dare you to try! Have fun with that!