To be fair, I’m using Linux, MacOS with Darwin Nix for managing it, Windows, and I still am not sure what exactly is an operating system, what’s the role of kernel and all of the possible system software is. Well, I think kernel is for hardware abstraction, but other than that ¯\_(ツ)_/¯
I would guess that it allows to detect tampering if you have to give your phone to the security officers and they do or don’t do something with it without you present. I heard of such occurrences on the border, but this happens in other places and countries, too. Not sure if locked bootloader would help, though