“[I]f you write the question [about a book] in the book’s margin—even if you just scrawl a question mark—you are marking the scene of your confusion. You are registering your puzzlement, not for the book’s sake but for your own sake. The interruption in the flow of your reading is a significant event, and you are quite literally taking note of it. Writing out the whole of your question is better than just flinging a question mark onto the page, because doing the former takes more time—it gets you out of the flow of mere passive reception—and because it forces you to articulate the precise nature of your vexation. A mere question mark could indicate confusion, disagreement, a feeling of lacking information—any one of a dozen things. When you write out your question you render the discomfort exactly.
This is important for two reasons. First, it sharpens your readerly attention now. Having formulated what’s bothering you, you have it clearly in your mind, and so when you return to the text you will be readier to note anything that answers your question or eliminates your confusion. Second, it’s a mnemonic device: the written-out question allows you at some later point to recapture what you were experiencing when you first read a passage—something that’s intrinsically interesting but also often quite useful.”
— Alan Jacobs, The Pleasures of Reading in an Age of Distraction