The DrayTek Vigor 120 is an ADSL modem with an Ethernet connection; it is not a router but a true ADSL Ethernet modem. By providing a PPPoE to PPPoA bridge, the connected device (firewall, router or PC) can log into the Internet (your ISP) directly and have full control over the ADSL connection - that makes the Vigor 120 a unique product. You can connect any device to the Vigor 120 which has a PPPoE client facility, which includes PCs, most Ethernet-WAN routers and the Apple Airport but the actual connection to your ISP is still PPPoA (unlike other modems which only provide PPPoE native bridging), which is the unique feature of this product and makes it compatible with all UK ISPs, where PPPoA is used as standard.
Other ADSL Ethernet 'modems' use workarounds to get a public IP address 'through' to your secondary device/client, requiring non-standard operation and complicated dual-stage setup (modem logs in, router routes) but the Vigor 120 provides the secondary device with a real routed IP connection and the ability to fully manage the connection, making setup easy. In most cases, the Vigor 120 needs no setup or configuration - just plug it in and set up your PC or router. All login/ISP details are entered on the connected client device, not the Vigor 120.
This method also differs from using a regular ADSL router which logs in itself and then uses NAT or multiple public IP addresses to create an onward client connection for your secondary device; that is not IP address thrifty, or introduces secondary NAT compromises. With the Vigor 120 bridge/modem, you get a true single public IP address (or multiple, if you have them) straight through to your firewall/router, which also has complete control of the ISP connection.