He does it to help visualize his mad thoughts about the game and keep track of small bugs and hot fixes (big ones need a big computer).
Does any one of you nerds is a game developer? Do you still use pen & paper to support your work? Do you still use pens or paper in 2020?
FUN FACT: the reason why we still use paper is because in 2012 we printed many, MANY, pre-filled receipts hoping to speed up the sales during a fair. Long story short, we didn’t sell anything and we were left with a ton of half used sheets of paper. Instead of throwing them away, we decide to “recycle” them using the white back of each paper.