It for places where you don't have or cannot run a full height chimney. A power vent only needs to terminate just outside the house. Since its forced draft, there is no need for a full height chimney. this is an option for many people without chimneys or cannot utililize existing chimneys.
I run both. My Hyfire uses the standard block chimney (35' high) and my keystoker uses a power vent, just thru the outside wall in the garage.
If you have a short chimney, without the power vent, you cannot create enough draft to pull all the fumes out of the stove and possibly leaking into the house thru the stove or baro damper as stated above.
With a standard chimney, the more heat you have, the higher the draft. Therefore, the baro will open more when the stove is really cranking or when its really cold outside, since heat rises faster if the temp is colder outside. My baro on my Hyfire is normally 1/4-1/2 open all the time due the good draft of the chimney.