It's definitely not to energize the coil. That's done by a PWM signal from the IPDM (which is how your car can run the voltage higher if the battery is dead to charge faster, etc). Usually the alternator fuse is there to make sure something doesn't dead short the alternator and burn it out. Honestly though if you're looking for an ignition on power source you need to use something behind an ignition relay. Usually the alternator and the in-line fuse for it are just run in series with the battery, so if your car shuts off you'll still have battery voltage...
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