If your car won’t start because it has a dead battery, use old-fashioned jumper cables. That’s what they are designed to do. The cigarette lighter cables do a different job. As has been mentioned above, they place a slow charge on the dead battery. Eventually you can start the car from its own battery while getting no assistance whatsoever from the helper car. It is a slow affair because the lighter’s wiring has limited current-carrying capacity.
The jacks in the rear seat are no different from a lighter socket. Possibly they are even less useful if their wiring is fused at a lower rating, which may be the case.