1. sudden road changes, like the appearance of potholes
2. avoiding puddles which can cause one side of your car to hydroplane
3. taking a 'rain line' when the road is wet. (Traffic tends to put slight ruts in the road. They're enough to make slight puddles. If you drive slightly off-center, your tires will be on the high spots and less likely to hydroplane, that's that 'rain line'.)
4. avoiding icy spots
The LIDAR system the car relies on doesn't work in rain, snow, or fog. Also, it would have a lot of trouble with the massive changes to roads and driving patterns caused by seasonal snow accumulation in the mid-west and north east.
It's much easier for the on-board processing to start with a baseline and then figure out how the world differs than to start from scratch from the sensors.
There are other issues, too, such as I've been behind gravel trucks that were constantly dropping gravel, and other vehicles with a clearly unstable load that I get away from. I also give wide berth to cars that don't appear to be driven in a stable manner.
It's relatively easy to create a first-time-through map of something. You simply record everything you see (or thought you saw). On revisiting, there are now questions of:
• Was that here the first time?
• Did it move? Was it mismeasured?
• What about the stuff that's recorded that we don't see?
Early cartogrophers (as well as researchers in conceptual fields) often mistakenly create or record features where none actually exist (or at least no subsequent evidence is found). Often the ghosts of these features haunt maps (or textbooks) for centuries.
Essentially, you need a map, ultimately, which has a sense for the permanence of any highway feature, and to not fall prey to map-territory relation assertion fallacies: http://en.wikipedia.org/wiki/Map-territory_relation
The map is not the territory.