This is less true for fully patched GrapheneOS devices than it is for fully patched iOS and other Android devices, but this space is basically a constantly evolving cat and mouse game. We don't get a press release when GrayKey or Cellebrite develop a new zero day, so defense in depth can be helpful even for hardened platforms like GOS.
I think, if you were actually willing to do that, it would probably be about as convenient and at least as effective to leave the device powered off and rely on the device full disk encryption and hardware security to protect the data at rest, only powering it on occasionally to check or send messages, then immediately powering back off.
Their justification here https://source.android.com/docs/security/features/encryption is that
> Upon boot, the user must provide their credentials before any part of the disk is accessible.
> While this is great for security, it means that most of the core functionality of the phone is not immediately available when users reboot their device. Because access to their data is protected behind their single user credential, features like alarms could not operate, accessibility services were unavailable, and phones could not receive calls.
I'm sure they could have found a better approach, instead of file based encryption, but must have been nice to simplify engineering overhead and giving 3 letter agencies, at the same time, something that simplifies their work.