NAT-Routers isolate and protect your private LAN (Local Area Network) from the public Internet for security purposes. As an added bonus, NAT-Routers will also typically include a Firewall to facilitate port blocking to prevent malicious port scanning by potential network intruders.
The NAT-router translates your public IP address (which is assigned to your router WAN port by the DSL/Cable Modem) to your private IP address(s) that your PC(s) and VoIP ATA adapter(s) use on your LAN. Thus, helping maintain your LAN privacy and security.
Now days, everyone should be using a NAT-Router as just one more tool in their security arsenal. As well as IP address translation and firewall port blocking, NAT-Routers typically have a built-in Ethernet switch to allow multiple PC's and devices to share your LAN as well as the Internet connection.
It used to be that many VoIP services had difficulty establishing or maintaining registration with VoIP ATA's or soft-phones because of the IP address translation process encountered when traversing your NAT-router. Thus, many services now use STUN servers which help the VoIP proxy server find your private IP addresses.
However, in recent times, I have found that with most of the SIP-based VoIP services I use, it is not necessary to fiddle with setting up STUN server configuration for my VoIP ATA's and some soft-phones.
With services like Flowroute, Voip.ms and CallCentric I have not needed to use the assistance of STUN servers.