By the time a node precommits, it should have the justification (POL) prevotes, the block itself, etc saved to disk so that should it restart, it can recall its lock state and prevote accordingly. Without this, validators can accidentally unlock upon restart right after a precommit, perhaps causing the blockchain to halt.

This isn't a problem unless a large number of valdiators go offline simultaneously. It could be a problem in the early stages of a blockchain when less than 4 validators are online, assuming equal voting powers.