They are probably non-elites for whom the agent would provide better service than United's low end contracted call center for non-elites (which probably has a wait time longer than the line at the agent during typical ORD delays).
The desk agents may or may not provide better service, but the hold time on the phone is much shorter than the 1-2 hour wait in those lines. I've had to deal with the offshore call center a number of times for irrops rebooking, and it's never taken me more than about 15 minutes on the phone to get fully rebooked. It helps that I look up alternative flights on the departure monitors while I'm on the phone, so I can offer those alternatives to the phone agent... for the general public, who may need hand-holding, a desk agent may be better, but the phone agents should be fine. The best tactic is to get in line and be on the phone while in line.