What belief in Santa at a young age taught me was a sense not just of wonder, but of Mystery - the delight in the idea that there are might be some things that exist that our rational universe cannot explain. It was a grain of the impossible in what was a very rational child. When I grew older, I understood that there is no Mystery, but previous belief allows me to dwell upon the delight of belief that possibly there was.
What science fiction or fantasy evokes is that old feeling. Deprived of Santa, I'm not sure it would have nearly as much attraction.
Of course, my wife hated the idea of being lied to by her parents upon discovery, and my children were far too logical for it to even be a possibility, so it Santa wasn't a factor for them. But for me, belief in Santa was definitely a plus.