VoIP main application is Long Distance – trunk replacement, meaning – replacement of traditional telephony trunks with alternative cheap bandwidth over internet protocol.
This revolution is absolutely transperent to you, the customer – accept maybe, the fact that the IP telephony led a “violent” revolution in the prices of long distance phone calls.
Giants like AT&T lose money as a consiquent of that very revolution.
If you still didn’t get it – you can make a call to your uncle in Germany from your telephone set to his telephone set through IP route and not even know about it. Only that the price you pay is much lower.