It also prevents the addition of content onto the disk, providing tamper-protection in addition to secrecy.
For example, spyware/remote-control software would open new routes to penetrate your employer's systems, or steal money from your bank account, bypassing other security measures like an additional layer of encryption on your password mode, or 2FA.
Even a relatively unsophisticated foe could connect your hard drive, dump incriminating data on it to frame you, put it back, and then call the authorities with a tip.
You are generally guarding against the device being shutdown.
Anything assuming the OS is running is already into "well that is easy" territory since if the OS is running those keys are in the same place as this attack leverages.
For example, spyware/remote-control software would open new routes to penetrate your employer's systems, or steal money from your bank account, bypassing other security measures like an additional layer of encryption on your password mode, or 2FA.
Even a relatively unsophisticated foe could connect your hard drive, dump incriminating data on it to frame you, put it back, and then call the authorities with a tip.