Why are parents so protective about their children's belief in Santa?

I know so many parents who get really weird about trying their best to keep children believing in Santa. They buy separate wrapping paper so the kids don’t catch on that Santa uses the same paper. Everyone talks in hushed tones to other adults about what Santa is getting the kids. It’s even been the plots of movies and television episodes. It’s extremely important to make tons of effort to keep the illusion of Santa alive.

I don’t think it’s worth the effort, myself. My husband and I both grew up with parents who told us very early on that Santa was a myth. My husband wasn’t bothered. I believed my parents were mistaken and believed anyway.
My daughter is four and we have also never bothered to keep any illusions about Santa alive. We have told her that he is imaginary. She still believes despite what we’ve told her. She thinks we aren’t very smart when it comes to Santa. She just knows he’s real. So we’ve got all the “benefits” of her believing and none of the effort that all the other parents I know put out.

Anyway, why is Santa so important?