The best thinking I've ever done was illegible, cramped into a quarter-inch strip of white space beside someone else's printed words.
Marginalia has a long and distinguished history. Dante Gabriel Rossetti filled the margins of his books with small sketches. Darwin annotated voraciously. Sylvia Plath left marks in her library books so vivid that scholars study them as primary sources.
Responding as a form of thought
Writing in the margins forces a particular kind of attention. You must compress your reaction into the available space. You must choose: is this surprise, agreement, argument? The physical constraint makes you decide.
I've started doing this digitally — not with annotation apps, which feel too deliberate, but with a separate notebook open beside whatever I'm reading. The effect is different but rhymes with the original.