Well, even if it's not clear that the company will survive a likely bankruptcy, it also makes no sense to stop attempts at innovation and operational improvements like what happened when the UAL merger was announced. I'm sure that management looked at the experiences of other airlines who implemented zone/group-based boarding (like AA, DL, UA) and decided that US could benefit from using zones to board as well.
As far as I'm aware at Delta, the zones don't correspond directly with the old way of boarding from the rear forward. They board First and Medallion in the first couple of zones, and the subsequent zones are staggered through the aircraft. I believe Zone 3 is at the back, Zone 4 is in the middle, Zone 5 is toward the front, Zone 6 is again towards the back, etc. I guess the idea is that you can board 3, 4, and 5 in quick succession and thus have 3 disparate congested areas. Then you board 6, 7, and 8 again in rapid succession. Hopefully, the different zones are staggered enough so that the congestion is minimized. I haven't yet figured out exactly how AA's zones work but they seem to be different from Delta's. The whole "zone" idea at Delta was tried out at Song; like food-for-sale, that's an idea that made it from Song to mainline pretty quickly.
The zones hopefully make things simpler for gate agents as well, since it's easier to look at a zone number on a boarding pass than a range of row numbers. Hopefully the passengers "get it" as well.