For the uninitiated, VoIP sends data over an internet connection rather than over cellular. Because international data plans can be quite expensive, this could appeal to US riders traveling abroad. The Verge also notes that VoIP option could come in handy for riders in locations teeming with people competing for cell service or where cellular reception is generally unreliable.
The VoIP addition might also appeal to drivers who prefer to keep all of their communications with their fares in-app. Another benefit of VoIP is privacy; neither rider nor driver is forced to use their personal phone number. While many riders in the US might not often see a reason to use VoIP to contact their drivers, Uber users in other countries and drivers who want to keep all Uber activities in one place may appreciate the new VoIP feature.