I think one of the key issues for Google, from a corporate strategy standpoint, is "freeing up people's time." Driving is one of last places where we spend significant time awake without being able to use the Internet and hence any of Google's services (except if you use a smartphone, which is now illegal in some states, and in any event isn't an ideal place to be clicking on ads). There is a safety issue here as well. Although using a smartphone while driving is illegal in some states, people are driving while using their smartphones with increasing frequency. We need our Internet "fix."
I am sure this driving technology also taps into several of Google's key capabilities: e.g., programming expertise, its voice recognition technologies, search, and its mapping software (Google Maps and Navigation).