FaceTime audio has been jokingly referred to since its WWDC 2013 introduction by some, as the phone. While this may be amusing to some, the wider picture is something potentially more important. In adding audio only calling to FaceTime, Apple has created their own Skype. And, with it, opened up VoIP to the masses.
VoIP calling is nothing new. We've had Skype for a long time, and it has become pretty much the global standard. BlackBerry too has voice calling within BBM -- the soon to be cross platform BBM -- so Apple isn't even the first to integrate a proprietary service. The downside to both of these, on iOS at least, is that they require a separate application. FaceTime is built in to iOS 7, which gives it an advantage the others will never have.
Every iOS 7 user will have acces to FaceTime audio. As such, all of your friends, your parents, grandparents, will all have a free VoIP service that they may not even recognise as such. Jargon is scary, and third-party services rely on the people you want to get in touch with having an account on the same service. If someone calls you on FaceTime audio, it's basically indistinguishable from a regular phone call.
How about telling your grandma in Australia she can call you from her iPhone completely free of charge, just the same as making a phone call? FaceTime audio is available right within the contact listing for anyone it's an option for. You don't have to worry if someone has it, because if they don't, you hit the phone call button instead. iOS 7 takes away the necessity for you to worry about finding out if someone has the same service as you, what their login name is, any of that. You can just pick up the phone, and call them.
So it really could be VoIP for the masses. You don't need yet another account and yet another third-party app. You just pick up your phone, find a contact, and call them. There is now a separate FaceTime app within iOS 7 on the iPhone, as there has been always on the iPad, but the same calling options are available from within the contacts, where FaceTime has always resided on the iPhone.
Apple had a similar kind of hit on instant messaging with iMessage. It's just there, you don't need to do anything special or download any special apps. And that's what matters to most average users. FaceTime audio isn't necessarily a Skype killer, but it's got huge potential in the space.