I've been told it's because they can get little bubbles in them from being jostled around, and letting them sit undisturbed for a day gives time for the bubbles to be re-absorbed. I don't think the air cell dislodging has anything to do with it, that's not going to re-attach if it's damaged.
Of course, it could just be nonsense. Not knowing for sure, I let mine settle for a while.
Of course, it could just be nonsense. Not knowing for sure, I let mine settle for a while.