3 Shut off valves under bathroom sink?

Quick question: has anyone seen a bathroom sink vanity with three shutoff valves underneath? I just saw one at a hotel in Atlanta and I was curoius why there's a third valve.

The first two run to the hot and cold water faucets, as expected. The third connects smack into the tailpipe/p-trap.

Anyone familiar with this?

The one thing that is sometimes run into a lav tailpiece is a condensate drain from an air conditioner, but I don't know why it would have a valve on it.

valve

Is there a floor drain in the room? If so that might be a tailpiece trap primer and the plumber did not know how to go from the tailpiece to the wall without using the valve as a reducer.

I think you might have got it hj, as there was a small floor drain under the vanity sink area. I'm still unclear on what the connection is between a tailpiece primer into the vanity trap and the floor drain. I thought primers were meant to keep the floor drain trap wet?