I write notes to solidify the idea in memory. Placing the idea in a real space helps me relate the ideas spatially. If I can remember one thing on a page, I can usually remember the rest of the page.
I guess it's the recitation and active recall that helps--not necessarily the writing/typing stuff down on paper itself. I suppose it's also possible to do this by drawing on a whiteboard or...eek...preparing and drilling flashcards.
