Why is there a suppy pipe to the drain valve?

I have not ever seen this type of installation before. Has anyone seen this or have a good reason why someone might install a cold water supply line to the drain pipe.

Thank you.

Where does the supply line lead to? Does it actually come off the cold supply?

It comes off of a cold supply in a mixing of pipes. It leads to a finished basment ceiling and the new owners have no idea. The utility room view was very limited.

I was there for a consultation and saw this.

Honestly It does not make sense to me either, because any sediment in the bottom of the tank can contaminate the potable water system. Or it could be some sort of drainage pipe for some other system. Interested in finding out what it’s used for as well.

I see them all of the time its a recirculating loop for instant hot water when there are faucet outlets a considerable distance from the hot water heater some times pumps installed in the line sometimes not

I did not find an instant hot water dispenser anywhere in the home.
Thank you for the responses.

A loop not a dispenser.

Look under the sink of the most remote fixture.

That appears to be plumbed as a recirc line.

It possible to have a gravity type recirc system with no pump if some care is taken in how it is plumbed. The other end of the line should be near the most remote fixture where near instant hot water is desired.