Why don't we give each other cash for Christmas? Your employer pays you cash instead of grocery carts full of stuff because you know what you want better than they do; why doesn't that go for gifts as well?
The usual answer is your Christmas gift is a signal, not just of your willingness to sacrifice cash for them, but also of how well you know them, to know what they want. But if so then why do we often make and distribute Christmas wish lists? Doesn't handing our free answer sheets defeat the purpose of testing them on how well they know us?
One answer might be that the gift receiver is like a teacher leaking answers to students to raise her teacher rating – maybe the gift receiver cares more that third parties think her gift givers know her well, than that they actually know her well. But in this case wouldn't she be trying to hide the fact that she passed around a wish list? If everyone who sees her get a gift was shown the wish list, who could she be fooling?
Could it all be an elaborate show so that she can honestly tell other folks that she got things she wanted? But if so wouldn't she be terribly embarrassed if they learned she gave out a wish list? That just doesn't ring true to me; what else could be going on?
Added: Registering for wedding gifts is a clearer puzzle; all are fully aware of the list, the list is very specific, and recipients don't save on shopping time/trouble, as they had to pick out the gifts.