My guess would be that someone thought it would work like a vent without actually venting.
It could also be for a planned expansion. Maybe a double sink? Doesnt make sense to do that, since PVC is pretty easy to work with.
Either way, whoever did it was most likely (hopefully) not a licensed plumber...
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The only place we've used them is a kitchen island where there is no possibility of a stack option. Local code doesn't allow a mechanical vent so we use these but never in a vanity. If your plumbing wasn't vented properly, a plumber could have added this to remedy a drain issue.I would elect to keep the loop if that's the case as opposed to a mechanical vent. Check your local plumbing codes. You could run into issues if and when you decide to sell.
I cannot wrap my head around how putting loop of PVC will vent anything.
Here is how a loop vent is made.
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I agree it's likely a venting alternative. I would rather use a ~$22 Air Admittance Valve (AAV). Home Depot, Lowes, and Ace all carry them.
If you want more reliable advice, I've always gotten really great responses from pros in both the plumbing and electrical forums on Houzz. Those folks are fantastic.
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