The comments above ring true. In the old days, if you found a cheap fare that originated in somewhere you went to regularly, then you bought the fare, say, from BKK to London return, and then just kept going, secure in the knowledge that you could always use the return half. Tickets bought in Thailand were cheap, don't know if they are now, and if you went there 4 times a year, or more -vinapu- , then this was a way of saving money!