Extra negative battery cable when charging outside car?
So that you can connect/disconnect while not standing over the battery. The batteries contain sulfuric acid which is not a very pleasant substance to splash. Even the fumes are dangerous.

While connecting inside a vehicle, connect the positive first and the negative on the vehicle body while not facing the battery. The negative can be connected to any exposed metal part on the vehicle (and at a distance from the battery). Similarly while disconnecting, disconnect the negative first and then the positive cable. The reason is that the vehicle body is connected to the negative terminal. If the positive cable slips and touches any other vehicle body part, it will cause a short.