Yes, it is for starting when the vehicle battery is dead. The issue is that the wire gauge is too small to carry the full starting current and the voltage drop will be so much that it won't start. So you have to hold it on long enough to charge the vehicle battery enough to start the engine from the vehicle battery plus what little current comes when the switch is held on.
How long? That depends on how dead the vehicle battery is, just hold the switch on and try to start the engine, if it doesn't turn over quickly turn off the key and wait and try again.