Most carrier level solutions have a requirement for a dual image system with multistage upgrade process aimed at reducing downtime of any single component in the network.
While very reliable there have been occasions where network engineers need to do a netboot recovery process after an upgrade. This requires a truck-roll to resolve even in the event that the router is operational.
If, like other systems, it were possible to roll back to a previous good image either manually or automatically under specific conditions during the upgrade process then it would be possible to reduce customer impact due to downtime required to get someone to the sit in order to manually intervene on the device.