There's a couple of benefits that come to mind almost immediately. First is the fact that those of us who have multiple phones and phone numbers will be more easily able to reach the driver and vice versa. Right now I use my Google Voice number but with changes to Google Voice it's not as easy to redirect the calls as it used to be to an international number (my GV number is enabled that way). Second, and more importantly, using WiFi to connect to a driver in some locations, like hotel lobbies, often has better connectivity than LTE or 3G or even 1xRTT depending on the carrier's coverage. Lastly is call quality. In most cases VoIP offers better audio.
Expect to see more mobile apps incorporate VoIP inside, either as SIP based calling or using WebRTC.