This is for an automobile battery (or truck and heavy equipment).
If you put a wrench on the battery positive terminal first, you might accidentally also allow it to touch some metal part that is connected to the chassis which is also connected to the battery negative terminal.
This is a direct battery short and can cause sparks, could weld the wrench to the metal making it difficult to remove and burn you as well as destroy the battery.
When disconnecting the battery negative terminal first, there is no problem if the wrench happens to touch anything else because most other metal parts are connected to the chassis and also that battery negative terminal. .. No harm, no foul.
Once the battery negative terminal is disconnected, there is now no problem if the wrench on the battery positive terminal should happen to touch any other chassis connected metal because the battery is disconnected.
If our battery terminal connect with the battery terminal connector, this will be more easer to operate.