>> Encryption is super important because it secures your data in case your machine is stolen.

> If your machine is stolen and off*

It's possible to configure macOS to wipe FileVault keys from memory when transitioning to a hibernation state. Assuming this works as designed, it might help in a snatch-and-grab situation if the thief closed the lid of the laptop. I would assume competent LEAs would take measures to keep their target's machine awake however.