According to the Wall Street Journal, the answer is simple: when airlines want to shore up their on-time percentage.

Delta Airlines flight # 815 from New York to Los Angeles now lists as a 7 hour and 4 minute flight.  The same flight used to take just over 6 hours.  This kind of “flight time inflation” seems to be rampant in the industry now, as airlines are padding their flight schedules so that late flights can still appear on time.

And it seems to be working.  In 2009, the major U.S. airlines recorded their best on-time performance since 2003.