Since self-driving cars are still a new experience for most people, all of that visible topside gear might give riders a sense of security.
This sort of add-on thing is temporary. Another generation and cars will have an equally good sensor suite, but it will barely show.
And plenty of taxis in my neck of the woods have rooftop advertising, anyway.
On the other hand, it makes it clear the car is an Uber. Illegal parking and such will be a lot easier to ticket.