[aerohead]

Without an 'outside' vantage point, there would be a region along the flanks, to the rear of the vehicle that would be 'invisible' to the driver.
You could potentially initiate a lane change maneuver while a vehicle behind you has already begun an overtaking maneuver in your blind spot.
I'm pretty certain that this is the reason why fresnel lens and camera systems have their 'source' vision beyond the outer surface of the doors.
The California State Driver's Training program admonished us high school students to never surprise another motorist, nor be surprised by another motorist.
Everyone needed a minimum three-seconds warning of driver 'intent' before ever turning the steering wheel in traffic.