Well, I board with my 53 year old sister with the "passengers needing assistance" call. She is retarded, and as a result, moves more slowly. She also has serious balance problems.

If we board with the general call, we would stall the entire line.

If we wait till everyone else boards, we are potentially holding up the flight's departure.

I will say, however, that I do enjoy the pre-boarding. It's easier to get overhead storage near the seat, and my hatred of lines kicks in too.