norton wrote:
Why ? What is the difference?
It removes more danger from the equation.
It's like when you connect jumper leads, if you connect the positive first holding the negative away from the receiving car, then there's nothing to short out on. You could connect the positive to the bumper and not cause an issue in the world, there's no chance of spark. Connect the negative first and as you bring the positive in you could clumsily strike an arc on anything around the engine bay or car destroying electronics, melting metal, ruining paint and possibly popping your own battery.
Same with switching the negative terminal from the battery off. Switch it off down the back of the car, and when you have a bingle then it doesn't matter if your battery cable gets pushed up against anything (like the exhaust pipe in the tunnel or crushed between the rear subframe and the car). You have removed the grounded chassis from the equation, and face it, this is where the real danger is. Anything can strike against the chassis anywhere in the car.