I know why we celebrate Christmas. I teach my children that the day is a celebration of the birth of Christ, God's gift (of salvation and so much more) to the world. It's a beautiful and important holiday to me. That gift wasn't small and it's eternal. There is no gift so great or so worthy of celebration.
Still, in our house Santa is part of the broader celebration. For my kids, Santa is a jolly man, with elves and a wonderful sort of magic, who celebrates the eve of the greatest gift by giving.
For me, Santa is the magic of wonder and whimsy and fun and the joys of giving and receiving. Santa is the spirit that makes even skeptics and non-believers make contributions so that others might happily celebrate those joys and know that magic. Santa is the spirit that inspires people to anonymously pay off thousands of dollars of layaways or deliver baskets of food or sing happy songs for people in need of company and cheer.
We cannot give the way God gives. But Santa's story is of just a person, slightly flawed but decidedly loving and spilling over with so much joy that it could not be kept within, but had to be given away for the delight of others. Santa is attainable--anyone can be Santa. So Santa can and does exist any place in which someone believes. And if I live to be 100 I will continue believe in Santa and the fun factor of Christmas.
My children have asked why Santa doesn't leave anything for me. I've explained that's it is because I have super potty mouth and a tendency to yell that really needs to be worked on. Since I'm trying to work on these things, I don't get coal or switches, so it's cool.