Commercial flights keep the atmosphere in the plane pretty darn arid during the flight to cut down on human stink and curb some transmission of diseases (possibly other reasons too). Subsequently, when you get off the plane you're going to be a little dehydrated. If you're picking somebody up from the airport they are really going to appreciate that water.

Alternatively, if you're taking a flight somewhere bring a bottle of water for during/after the flight.

You generally can't bring your own bottle of water through TSA. You can bring an empty bottle through with you, then fill it at a drinking fountain.

(AW- Anil)