Every other computer behind your gateway will have an IP address of 192.168.0. As each of these computers make a connection on the Internet, it will show the external IP of your gateway. So the benefit is that your internal LAN is hidden from the Internet. A firewall blocks requests to your computer. You may want to have FTP running for instance, but don't want the rest of the world to be able to access it.
Some people think that once they've got a firewall they're secure.
That's not true. If you've got a firewall and have a webserver running for instance, you're not secure. If a bug or security hole is found in the webserver you use, it could be used to crack into your system. So keep that in mind. A firewall does help, but it's not going to keep you completely protected.