Originally Posted by StewieMac
Actually, it's used because in LGW international and domestic passengers mix freely after security.
As you do not need to have photo ID for domestic flights (in the UK), they use the digital photos to make sure that you're the same person getting on the plane who went through security (no need to show photo ID on boarding).
Quite what purpose this serves, no-one ever explained to me...
The purpose behind it used to be that domestic passengers could mix with people arriving from "dirty" countries. Since LGW has now completed the segregation of arriving international passengers, and the new domestic arrivals baggage belt is also sgregated from departing passengers (in case someone opened up a suitcase and handed over something like scissors to a departing passenger) there seems to be liitle point in keeping the system.