Encrypted firmware is harder to modify. If a hacker wants to inject malicious code into the kernel before it loads, they must first decrypt the firmware, modify it, and then re-encrypt it—which is impossible without the proprietary key. This creates a "chain of trust" from the BootROM to the kernel.
Public repositories of firmware keys around iOS 10/iOS 11. ios firmware keys
: For older, encrypted components, a hardware-level key called the GID (Group ID) Key Encrypted firmware is harder to modify