For one thing, Google gets employees who can work during their commute without getting shouted down by San Francisco renters for putting them all on a shuttle bus. For another, they'll probably license the technology to any and all car manufacturers on a per-unit basis.
Third, and most important to this article, if you're not driving then it is safe to use the phone. You can talk on it, read from it, edit things on it, and otherwise use Google's software on it.
Fourth, you can spend more time that you're not using the phone or laptop to read targeted Google ads on video billboards.