1- theft risk/reward is crushed. It’s simply no longer feasible for stolen iPhones to be parted out if the valuable bits don’t simply work. Sure, dumb and non networked components like frames and glass can probably be salvaged, but when even batteries are involved in the handshake process, you lose out on the ability to sell anything of value.
2- positive supply-chain validation. Not important for the majority of people, but for those who require a little more security, they can be a little more sure that their device isn’t compromised from illegitimate parts. I imagine this to be a fringe benefit for executives and the like. I know at one point government officials had access to some “special” variants of iPhones which were more locked down, but specifics are difficult to come by.
For everybody else, this plain sucks. We move farther and farther into not even owning the physical things in our possession.