From a newly published page on Apple.com:
Apple is driving vehicles around the world to collect data which will be used to improve Apple Maps. Some of this data will be published in future Apple Maps updates.
We are committed to protecting your privacy while collecting this data. For example, we will blur faces and license plates on collected images prior to publication.
The page also displays a list of cities that these vehicles will be driving in next.
There’s a lot of speculation that Apple is going to be using this data to introduce a street view-like feature, but I think we’ll be seeing these images first appear at the top of business listings in Apple Maps. Currently, these images are being provided by Yelp and are often of poor quality.