Some flights I never even saw the BF concierge, on some others s/he made rounds in the cabin, addressing passengers by name, offering thanks for the business. A couple times the concierge brokered some seat switching to allow families to sit closer together, as in an IAH EZE flight. The CDG concierge arranged access to an arrivals lounge with showers, and on another occasion helped with seating an PC access. The HKG concierge helped get a seating mess resolved.
My guess is that they do several things behind the scenes that don't get noticed unless something goes wrong. It's kind of like nurses in hospitals - if they are doing a good job, bad things don't happen - but who notices what doesn't happen?