Why? Because there will come a time when your motivation wanes, and then you'll not feel as fired up, and ideas don't come to you as readily. This is not the time to try remembering what came to you when you were feeling good. A short pencil is better than a long memory, and strike the hammer while the iron is hot.
I Use A Pencil: The Experimenter's tool
Yeah, a pencil gives me freedom to experiment. A pencil is great for what's at it's butt---the eraser. We are not at the commitment stage yet, so let's feel free to try things out---check if a sentence is good or not, and use the eraser if not. A pen puts an indelible mark on the work, and makes changes messy---not time for that yet. On the bus, at the gas station, on the flight, I can quickly pull out a pencil and be off to writing.
Let's write.