I equate the concierge to a "table captain" at a 5 star restaurant. They're there basically to make sure everything flows smoothly during the boarding process, and insure every guest is treated well and remembers the experience. On my trip to LGW in Jan, I was offered a shower, and they knew who I was when I walked in the door at the PC( I was the last one to arrive). On the way back, I don't recall one being offered.

I wouldn't be surprised if they are the final person to assure all necessary service items are on the plane. (or at least sign that they are)

And I'm sure maintaining the integrity of the cabin is a part of making the experience memorable