?
I take a shot at trying to explain. Keep in mind, this is the way I view it. Everyone may not agree.
If you have ever looked into the inlet of the exhaust housing, you will see that it botttle necks down to a hole less than 2 in. in dia. With a production style wastegate, all the exhaust that comes out of the 6 cyl. must pass through this opening.
With an external gate all exhaust gas still passes through this opening until the desired boost level is reached, then the external gate opens, allowing this exhaust to exit prior to the turbo, thus better breathing.
IMO, this is the main advantage. Another is the ability to adjust boost from inside of car, or where ever your controller is mounted. From my experience, boost is more constant ( steady )
You are taking the boost control away from your chip and wastegate solenoid. Now you can control it with a simple turn of a knob.
The down side is buying a gate, mounting it and if so desired, piping back into exhaust system. If I/m forgetting some thing, someone jump in.