It's an anti-cheating thing.
Say there are a couple jokers at your table and they are in a lot of pots, always when there is a 3rd player involved. They might be trying to team up on the 3rd guy and sharing the profits/losses. Eventually you get to the river and one of them must show, but the other guy can muck his crappy hand and you wouldnt be able to tell if he was playing garbage or not.
etc.
