One gives you a quick win. The other gives you the C2 certificate.
But what did you learn? Nothing about why we invert after "had," or when it’s preferable to "If I had known." On exam day (IELTS, Cambridge Advanced, or Proficiency), no one gives you a multiple-choice answer sheet. You have to produce that grammar in writing and speech. My Grammar Lab C1 C2 Answers
Next time you reach for the answer key, ask yourself: Do I want to be right, or do I want to be fluent? One gives you a quick win