Like most aspects in life, there are right ways to do handle things and wrong ways.
Neck cracking is not inherently bad. It can be harmful if done improperly or too often. This is why it is a necessity to have a qualified professional perform the neck cracking.
Cracking your neck forcefully can pinch your nerves, making it impossible or painful to move. Cracking it too hard can strain the muscles around your joints.
Needing to crack your neck can be caused by hypermobility.
This happens when the joint has a larger range of motion than normal. When you give in to the urge, the ligaments on your neck can be permanently stretched. This case is called perpetual instability and may lead to osteoarthritis.
Remember, you can find a lot of blood vessels on your neck, and cracking it too hard can puncture these blood vessels which can result to blood clotting which can be dangerous since it can block the blood flow to your brain.