Comment by mnw21cam

The simplest answer is that the journal size isn't infinite, and not everything goes into the journal (like often actual file data). Therefore, stuff must be removed from the journal at some point. The filesystem only removes stuff from the journal once it has a clear message from the drive that the data that has been written elsewhere is safe and secure. If the drive lies about that, then the filesystem may overwrite part of the journal that it thinks is no longer needed, and the drive may write that journal-overwrite before it writes the long-term representation. That's how you get filesystem corruption.