Because there’s nothing stock about my car?
But seriously, I wanted to get rid of the entire alternator/bulkhead/charging bypass that you have to do with the original wire harness with any kind of significant amperage.
My car has its battery relocated to its trunk, I run electric fans that draw like 35+ amps running and more on initial start up, an air/fuel gauge, aftermarket stereo, and have plans to go EFI at some point. Using a harness that has provisions for most of that stuff built in makes a lot more sense than buying a new OE style harness that has no provisions for ANY of that.
