From the 'benefit' of driving an SUV / estate with the offending windows.
You can see following traffic clearly when the outside is brighter than the inside.
People walking passed it when parked can't see what goodies you have in the back.
The down side is reversing in a very dark place is nearly impossible without a camera and only the door mirrors will find cyclists riding with no lights.
On balance privacy glass is a pain I could live without..