I belive what you are talking about an air gap fitting, something like this (sorry, that was the best picture I could find, the same site has some good information about backflow prevention though.)
Plumbing code varies but normally anytime the water supply can physically contact the waste water some sort of device is required to prevent waste water from being ‘sucked’ into the water supply line in the event of a severe water pressure drop in the supply line. There are various backflow prevention devices to do this but a an ‘air gap’ is probably the best and most reliable.
A normal bathroom sink has an air gap because the sink will overflow before it reaches the spout, therefore there is no connetion between the water supply and waste water. On the other hand an outdoor hose can easily be placed into a bucket and contaminate the water supply in the event of a vacuum in the water supply, therefore in most places outdoor faucets are required to have a built in backflow prevention device.
I would guess in the case of a grocery store freezer there is probably a hose in the freezer for cleaning purposes or some sort of built in spray, the airgap prevents the hose from a possible connection to the waste water. I have also seen these fittings on the drains for commercial sinks where a hose can be dropped into the water.