Well.. if everything goes well, you'll never need it, but if for some reason your battery gets substantially drained and you fire your car up, the new high amp alternator is going want to do what it does best: charge the battery as fast as it can. The stock wiring can't really hold that much amperage before melting, so I added that 8 gauge wire just in case of that very scenario. I just ran the wire along the wiring harness, inside the cockpit, and then out on the other side of the firewall down to the battery connection on the starter.
There's only one disadvantage really, my ammeter now only reads discharge, since I'm effectively bypassing it with the 8 gauge wire.
I've heard some people say that they don't add the second wire and just make sure that the battery is chargedbefore starting the car, but unfortunately, you can't always guarantee that you'll have a way to charge the battery before running the car.