I wrote firmware on paper for years while drinking wine in a bar. I wrote some of my best code and fixed some of my hardest bugs while doing that. You can tear paper, tape it together. It can get filthy. You can use all kinds of pens. It is cheap and you can create any size overview you want. And you can look important and get asked questions. You can ruminate on problems for longer time such that you are 99% sure it will work.