Say that someone steals my computer or gains access. If it shuts down and everything is encrypted, it would stop the information leak that would cause, perhaps prevent some 2-factor, email + password reset or e-mail change sort of hacks into random accounts that I may have.
I want to point out that if your computer is "on" (and unencrypted) when an adversary gains access to your computer, your adversary may be able to obtain the decryption keys that are being stored in RAM (memory). If an adversary gains access to your decryption keys, they can later bypass any full disk encryption you are using fairly trivially.
This means if your computer is "on" (and your hard drive is unencrypted), you will need to be warned long enough in advance for a) you to react to the warning to understand what is happening and start (and) b) either cutoff power or shutdown your computer (which may involve physically removing the battery if using a laptop) before an adversary will be in a position to physically separate you from your computer.