Water
Considering that fresh water is a scarce resource in many places, I think the best use for captured water vapor would be to increase that supply.
I think of all of the clean water that comes from the air conditioners in homes and office buildings, which goes into the sewer system where it's mixed with sewage, then cleaned up at a water treatment plant, then released, and where I live it goes very quickly into a large body of salt water.
Maybe the amount of water reclaimed from the atmosphere is insignificant, but on a hot, humid summer day in Houston, I would think we'd be pulling lakes full of water out of the air.
