It's a pressure relief valve to equalise the atmospheric pressure inside your panniers with that outside them when you change altitude over a short period of time. Beware the sharp bit inside the panniers - it's been know to eat helmets.
Clever aren't they? (The Bmw designers) On a Honda they can't even make the panniers waterproof so no pressure relief problems for them ��
BMW must have decided they weren't really necessary cos my 2012 panniers don't have them.
(That said, as they don't seem to have too much trouble letting water in, I doubt air would be a problem...)