HJC gives you precise control of your curve radius and rotation rate about the bar.
Most approach calculators make you choose what stride you will start your curve on.
To clear the bar efficiently, you must control the rotation rate about the bar exactly, which means that you must control your curve radius exactly. HJC gives you that control.
HJC preserves your run length regardless of what other approach parameters you change. You don't need to fish for your starting point each time you adjust your approach.