Why long spanner is used instead of short spaneer to undo a bolt?

It is called leverage. Suppose you have a 1/2" bolt and your tool is 6" long: you have a 24 to 1 mechanical advantage, meaning one pound force on the spanner give 24 pounds force on the bolt. One pound times 6" radius gives 6 inch-pounds of torque, and when that acts on 1/4" radius it produces 24 pounds of force. You can slip a steel pipe over the spanner to increase the force even more.