Music, Nav, Video, and Phone connectivity through the iPad's microphone (or an extended attachment). If I ever need to play a game, or email someone while driving its nice to have that option which the iPad will allow. Its that simple.
In response to tablet PC's being more powerful or more useful, they fail IMO. Why? because every tablet since the beginning is plagued by one major problem, PC companies thinking they can run full fledged OS's on them. Even from demos of the up and coming HP Slate showed its sluggishness. Some people can deal with that, others can't and shouldn't especially when driving.
Final point: Turning this into some type of iPad Vs CarPC war is not needed. Heck, with the lack of publicity the CarPC gets in the industry it wouldn't even be fair. The iPad is coming like it or not, and you can be sure its going to be mounted in cars. The question is, will producers figure out a safe way to implement the devices and software or just leave it up to customers to try DIY methods. You can imagine all the weird and unsafe places people will mount these (in the windshield, on their passenger seat, steering wheel, glove department, visors, laps, etc.).