If left unattended, an attacker could install bootable code that impersonates your normal startup sequence, prompting for your encryption key and booting as nor

If left unattended, an attacker could install bootable code that impersonates your normal startup sequence, prompting for your encryption key and booting as normal, but secretly storing or forwarding the key to the attacker. (I believe this is an example of an evil maid attack, although that name for it is somewhat new to me.) A locked bootloader would prevent such code from being installed, or at least refuse to run it.