Not too much of a hardship, although the point has never been explained, a security issue of some sort perhaps. The photographs are destroyed once the flight has departed (allegedly).
It's commercially driven, making sure domestic, international and transit passengers share a common usage lounge means there has to be some check to prevent anyone swapping tickets and a transit passenger with no right to enter the UK boarding a domestic and getting off without clearing passport control. (Transit passengers are not subject to immigration controls unlike the US).
Rather than insist we all carry passports, they stop you at security and take your picture, however this is another barrier to progress, forgivable at a large hub but dumb at MAN where thousands of people every day are subject to it for the benefit of a handful, literally a smidgen of Intl-Intl transit passengers.