Robo cars are more expensive during peak usage, so they can form habits where public transit is both encouraged because I took a robocalled to work and the bus home because then it was rush hour, or Vice versa. Or I took the bus to work and left late at night after the buses stopped running via a robo taxi…etc. All this occurs with Uber already, I don’t bother renting a car because even though it sucks to take the bus from the airport, I can Uber that and use more efficient transit when available, since I have something that fills in the gaps. Robo cars just make that link more efficient (lower cost eventually as humans become more and more expensive).
They make not taking your car with you a bit more feasible, and that’s a start at least.
Traffic isn't a function of road space. The true throughput of roads is immense when utilized by better drivers (one extreme example: https://www.youtube.com/watch?v=4pbAI40dK0A). A lot of traffic is caused by bad human drivers: accidents (and associated rubbernecking), shockwaves, running red lights, double parking, circling around looking for parking. Additionally entire additional lanes of traffic (for cars or personal mobility vehicles) will be available when streetside parking isn't necessary. Personally I look forward to the possibilities of turning all that unecessary parking space into something useful ("we’re the Saudi Arabia of developable land in cities").
They make not taking your car with you a bit more feasible, and that’s a start at least.