I think of it as a challenge for the child to realize reality. To notice if what they're doing or believe is a lie or not. You learn to believe in only what you can back with reality.
For me, i stopped believing in santa when i learned enough about computers that norad started me thinking about the horrible animated quality, and about the bullshit equation of how many houses he would have to visit in the allotted time that it would be night and delivery. And possibly the picture of the people standing, GPS-supported, at the physical north pole, with no "santa's workshop" might have tipped me off. don't ask where i saw it, i don't know.
I simply understood later why they let me believe that for such a long time, and it also taught me to think for myself moreso than i had ever before.