It’s a general guideline that’s accurate for average people that live average lives, but if you’re too tall, too short, too muscled, have lower or higher bone density, etc it can be very inaccurate. No doctor takes any action on this alone, but it can be a guideline to ask you for blood works or other studies to ensure things are okay. There are better ways to measure body fat percentage, but that requires special instruments and also are usually dependent on where your body stores fat, so you might not have ever done them before.