From pages 97-98 of Opening Up by James Pennebaker:
Many times, when we write something down, we don't have to think about it any longer. I've noticed this in myself when I'm preparing to go on a vacation. There's packing, stopping The mail and newspapers, getting the car checked, and on and on. In the middle of meetings or talking to someone on the phone, overlooked chores come to mind: "Oh, I can't forget to pack the fishing rod," or "Get someone to water the plants." As much as I try to avoid it, I usually break down and start making lengthy lists of last-minute tasks to perform. Before list making, I actively juggle the tasks in my mind. Once I start the lists, however, my mind becomes freer and I feel less distressed. I have, in essence, transferred my mental notes from my head onto a piece of paper.
This is a pretty good summary of GTD from an experimental psychologist.