Not very much at all, IMO. In the case of a punctured primer the indentation of the striker is the source of the leak and the gas is going back through the striker hole in the breech face into the action. Those channels might bleed off gas in the case of leakage around the outside of the primer.
I’m guessing Stan is on the right track, mostly because I have so many guns that don’t have them, and I’ve walked away from a punctured primer or three without injury.