Much, much more than traffic, though that is useful. The anonymized probe data is used to refine business driveways inferred from satellite imagery, for example. That’s why suddenly Maps can often route you to the correct parking lot instead of a nearby curb. Think about it: if you know a phone is navigating to Safeway, where the user stops navigation is potentially interesting in aggregate and divulges almost nothing except the average parking preference of an iOS user.
Source: Worked on that. One example of hundreds.