It's an opportunity to pick up some room keys by trading.

A round robin starts with 100 or so keys and is mailed from person to person. You can take what keys out that you need by replacing each one with one from your extras making sure not to duplicate any keys in the package already. The package makes it way around the country until everyone on the list has seen and gone through it and then it's returned to the original person who started it. It's a neat way to pick up some needs for the cost of postage.