You write something - a chapter, perhaps, or a short story. Then decide it's not in the right sequence, so you have to add bits, re-phrase sentences, move paragraphs around...
When I did the dissertation for my Master's degree, circa 1980, it was shortly before home computers were around, so I had long trails of paper hanging down the curtains, all sellotaped together in two-metre-long streamers. The scissors flew; red and blue markers had arrows criss-crossing the whole papered area.
Now we have computers, and I need a screen the size of my front windows - about four metres wide by two high. I can't shuffle big blocks of writing on a 30" screen, or even three of them linked together, as a friend has.
So I'm still in the dark ages sometimes, as with this one I was sorting out last weekend for one of the "Realms of Kyre" Medieval Fantasy books. It did turn out fine - logical order, dramatic dialogue, correctly spelled, and with my idea of good grammar, considering the speakers are medieval-type peasants and lords.
So, of course, I did have to use the computer for the final type-up.