Most of the comments in this section apply to US domestic flights only.
Airlines tend to overbook their flights in case of no-shows.
Occasionally this will mean that more people show up with confirmed reservations than there are seats on the plane. The airline will then put you on the next available flight to your destination, along with your free ticket. Vouchers are typically valid for only a year from date of issue. If your voucher is expiring you can also try using it to get a ticket with a particular origin and destination but no specific departure and return dates. Such tickets should be good for 12 months.