The one big use for these as a globalist that I've come up with is guaranteed upgrade to a club level room (not just club access). For example at the HR Paris Etoile, the base rooms have occupancy of 2 and the club rooms have occupancy of 3. So you can book the base room (cash or points), and apply a club certificate to get a room with an occupancy of 3. You would have been entitled to the upgrade if it was available at check-in, but sometimes you don't want to take that risk.