I will go even further. The physical sensation of pen on paper ... including the texture and pressure you apply to key words, capitalization, underlines, etc ... all if it being fed back though your muscles into your brain and getting processed/stored/intertwined with those vey ideas and thoughts ... and the mental faculties you allocate to aligning the text against the margins, ensuring neat spacing, etc ... puts your brain in a zone better tuned to the task at hand IMO.
Some of it may be just ... overload your brain so it cannot think of anything else ..so you stay focussed for lack of choice.
But writing enables arrows, lines, crossing-out, small-text notes, circling, variable pressure, colour, etc. Richer than ordering/indenting text. Also higher contrast.