GTD has the addition that you must create a system of reminders/followups. GTD is great to practice being okay with forgetting stuff and trusting your tracking system.
I don't disagree that it performs that function. In my experience, thoughts will persist and remain on my mind until I consciously refine them enough. Writing is one of the best ways (if not the best) for doing so.
Writing down things makes it much easier to move forward to the next project of the day.
Probably various a bit from person to person.