An External Hard Drive for Your Brain
Your notebook is like an extra brain. It's not a place to store ideas that won't fit in your head; it's a place to store all the ideas that you can't use right now but want to save for later. You might be thinking: “Well, if it's a good idea, I'll remember it. I don't have to write it down.” And that's exactly the reason that you do need to write it down — it's a good idea! You will forget your ideas, no matter how good they are. That's just the way it is. You can't rely on the unlikely chance that you will instantly recall an idea when you need it.
As a comedian, there is nothing like looking through your notebook and laughing out loud at something you wrote down months before and then completely forgot about. Why is it funnier to you now than it was before? Maybe you were just too close to it before, and now you see it with a fresh set of eyes.
Also, the idea might not seem all that viable at first; it might take some time and distance to make it better. Do you think you'll have instant access to these alternative ideas as well? How about all the thoughts that aren't even a full idea yet? Probably not. If you want to be a successful comedy writer, you really need to develop a system for keeping track of your thoughts.
