This is the purpose of those airplane seat hooks
You have probably been staring at it without even recognizing it. Do you know how there is a little clasp on your tray table to keep the tray locked upright? Occasionally, a little hook is incorporated into that lock. And although not everyone wears a blazer or suit on an aircraft, when they do, they often have difficulty storing it.
Those hooks are included for the purpose of hanging your outerwear. Who would have guessed? While they are perfect for a jacket or cardigan, they are not as effective for a big coat. The coat may slip off, or you lose valuable legroom due to a large garment hanging in front of you. Moreover, you can use those little hooks to hang your headphones.