Say you find yourself in the middle of nowhere with a dead battery and no way to roll-start your vehicle. What do you do? If you happen to have enough rope lying around (or belts, for that matter), you can conceivably lift the drive axles, wrap the rope around a tire and pull with all of your might with the transmission in gear and the ignition on. The motion should be enough to turn the engine over and start the combustion cycle.
A few safety precautions here. First, make well sure that you can release the rope as soon as the engine fires, or you'll quickly be introduced to the front fender well. Second, don't waste your effort on an automatic. Finally, be warned that it's going to take a significantly larger amount of effort (thanks, compression!) for any vehicle with more than four cylinders.
Although I find this trick is quite handy, I do believe pushing and engaged a gear is more practical. At least for manual transmission car.
