Sorry, I got the barrels muddled that's the ignition one and not the glove box one
Could it serve to trigger the rear spring? ie you pull the key, which releases that latch, allowing the barrel to pop out slightly
Nope, it couldn't do that, as the only pressure from the spring is radially outwards against a smooth level annular ring in the outer barrel. It must be that as the tumbler cylinder is pushed into the barrel, the inner, flat, face of the latch comes up against the inner annular step in the barrel, and ensures that when the key is turned, the tumbler section rotates in the correct plane. So that once reassembled the whole unit is held forward in place by the catch which is released on removal, and the little devil we are discussing ensures that a) the tumbler cylinder cannot go further in, but more importantly, b) that it rotates, on turning the key, in the correct plane. However, as I said before, the only argument against this hypothesis is that the same could be achieved with a fixed lug on the barrel