I did bug fixes and used an in-house developed mapping system long ago--in Fortran on a VAX, so yeah, my experience isn't the most current...
I would assume it is also sending updated "I'm here" information to the server as needed for recalculating routes, etc. And that data can also be used to help improve the maps.
Assuming the road data you have is like the black line, but all the location data from people traveling on the road is the red dots--the servers can start to identify a difference. (They would get some sort of speed with the dots too, so you could tell the data was from something moving at the speed of a car rather than a row of houses).
If I was doing it, the servers could detect that sort of discrepancy and add it to a list to be reviewed by a person. Though maybe the server could update automatically too. (Automatic updates would be a question for all the ex-Google maps people that Apple is hiring :)
But to do that you need data. That's what Apple didn't have before, and was even giving that data to Google. But now they've got 100 million people providing that sort of information to them.