When you promise something, you open up possibilities.
The path of promise
When you commit with the firm intent to keep your promise, you set things in motion. You open doors of opportunity; you bring resources and help to your side that are beyond chance and personal control.
You also close the door to other possibilities. This helps you to focus all your energies on the course that you have chosen, rather than leaking your power in every direction. And it precludes all kinds of dire consequences.