Are you/they travelling to Spain? I've just been in a similar position myself flying to Barcelona, so did some research.
Since mid 2007 the Spanish authorities require this info before you can travel. Some airlines (like Jet2) collect this at check-in and transmit it electronically before the flight takes off, others (like EasyJet) collect it at the time of booking and send it by other means. If you look on the airlines website there will be something (hidden away) to tell you about this.