"When airlines started frequent flier programs in 1981, they just wanted to identify their top customers to market to them most effectively. In the early years of these pioneering loyalty programs, airlines knew only who went where and how often. People volunteered their information gladly to receive free flights and perks. Over time, airline officials wanted to know more so they started buying outside information about their millions of clients.
By combining the information they know about where you travel with outside commercial data from data brokers such as Acxiom or Experian, airlines today have more intimate insights on their travelers than ever before."
Read "What United Airlines Knows About You And What They Don't Want To Know" - from Forbes