1. Improved visibility: A human driver sits in the driver seat, and looks at the outside world from inside the car - partly through mirrors, partly with the view obscured. A self-driving car (at last current potential vehicles) has a view without being obstructed by the car's frame.
2. No limitation on focus: As a human driver, I can look to the front, or the back, or the sides, but I can't look everywhere all the time. I can do my best to maintain an overview of my surroundings, but need to update this constantly by changing where I'm looking - if I want to see if there's someone behind me, I (for a moment) have to stop looking where I'm going. Automated vehicles don't need to stop looking to the front to see to their back or the side. They can see a car 200 feet away coming from the right without having to miss the pedestrian coming from the left.
I'm not saying that self-proclaimed self-driving cars do this already, but looking at perception capabilities, self-driving cars have the potential to avoid more accidents than humans.
However I just wanted to say that a good driver can still avoid crashes even if not 'at fault' by driving defensively.
A friend of mine is a bad driver (in my opinion) but has never had an at fault accident. They have however had plenty of not-at-fault accidents but we're probably somewhat to blame or could have avoided if being more careful.