They are used to update old plumbing or adding a fixture somewhere you can't put a vent.
You can put the vent out the front wall and elbow up just like the chimney. My old neighbor, a high end contractor and licenced plumber, did that for his pahs underground house.
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Mine is simply tucked under the counter in my bathroom. It is a little far away from the tub so I still get a bit of gurgling when that drains, but works well for the bathroom and kitchen sinks. I really was not wanting to run a vent pipe up through my cob wall or have it run adjacent and be visible so I did some research and found these little beauties. Just a one way valve that lets the air in but no stink in.