<disclaimer> I don't have a WSM any longer, and did not use a billows with it.
I use the billows on the BGE for "Temperature Insurance" for overnight smoking.
I attach the billows, use the damper as part of the temp management, but I do not plug in the billows until the pit temp is stable, meat is on and again pit temp is stable.
Since it is already stable, I plug the billows in to the SmokeX4 and set the fan 5 degrees below my current target temp, and then I go to bed. If the temp drops ( It usually doesn't ) then the fan can maintain a good temp. If the temp gets too high ( It usually doesn't ) then the alarm will go off that the temp is out of range.
It isn't necessary to have, but I like it for overnight cooks as a safety net.