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.
Previously to the Biometric face scanners being introduced, any Intl-Intl pax were escorted to the gate by a security agent and were handed over to the Dispatcher/Gate staff. - A much more sensible approach for the small number of Intl-Intl transfer pax. However, the Dft in their endless quest to justify themselves insisted on the whole Biometric face recognition scanners at great cost to the airport and as Skipness implies, another inconvenience to the majority of travelers through T3 at MAN.