It possibly saves them being identified (or misidentified) by the Met's image recognition software. Did I see 96% error rate quoted somewhere?
Face masks do not necessarily prevent identification. This story
https://www.theregister.com/2020/06/17/facile_recognition_feds/ describes how the FBI identified a rioter from a tattoo in one picture, a T shirt in another and comments on the T shirt left on a review.