First, this is just a trial, they'll get a lot of information about how useful it is and tweak it accordingly.
Second, consider the data generated. A little bit of number crunching will generate a lot of highly useful information that can be used independently. Such as a map of all parking areas in the city along with other metadata such as how often there are spaces available, etc. Think about how this can be used. It can be used to provide better driving directions so that you are directed to nearby parking structures instead of the destination itself, when there isn't parking at the destination. It can also be used to figure out where it's best to park based on factors like general location, time of day, day of week, etc.
Now if Google could collect real-time data via automated parking ticket systems, or even by getting access to security camera feeds and doing some image processing, then they'd be in business. I don't know how these systems are typically run, but it might be a smart move for Google to pursue partnerships with popular big-city parking locations for exclusive access to this data, to the extent that it can be collected from current systems.