It puts iPhone in cold boot state if for instance police or any agency confiscate it from you. Better tamper proofing.
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[0] - Which I find deeply ironic, in that merely a decade ago, people would laugh at Windows with its "reboot after installs, reboot in case of problems" approach, and now frequent reboots are seen as Standard Security and Stability Practice on *nix systems, both mobile and server-bound...
It’s not like that anymore in my experience at least but the stigma stuck.