Excellent question. This would also require not having it in your profile. When you create the reservation, it appears to be sucked in from the Account (I'm pretty sure we did not enter it). So, something goes into the record from the Account.

And then, from what I'm told, the link is lost and updates to Account will not make it to existing reservations (although another current thread has a different experience).

But I don't believe it's required for ticketing.

Cheers.