Maybe he likes Frasier and wants to see where the street scenes were shot. He's not willing to pay a whole lot, but if a good deal comes along, he'll pack his bag and go. The airline knows that Mr. Online is not likely to hang around the airport, and they know Ms. Inline isn't likely to be surfing the Web. So, they can charge two different prices to these very different customers. The seller does well financially by practicing price discrimination, but so does society.
Price discrimination makes the airline wealthier, but it also does a service for society.
At the end of the day, both seats are filled, both passengers are happy to be going to Seattle, and the airline's shareholders are happy. If so, one seat will go unfilled, and Mr. Online won't get his trip to Seattle. We see price discrimination in many economic activities, and people do benefit. Doctors traditionally charged lower fees to poorer patients. Insisting that all buyers pay the same prices would yield some highly unpleasant results.