With more and more people connected to broadband, both high profile corporates and home users need to ensure that their networks are secure against external Internet attacks. By having a broadband connection you are always connected to the internet, and without a firewall to protect you, you could be at risk from hackers who regularly scan the internet for computers with no security software.
Attacks on high profile corporates are becoming more common and any company at the receiving end of such an attack would normally need to spend many hours, if not days, recovering from it, suffering losses both financially and in terms of productivity. These high end dedicated firewalls ensure your network is protected by providing additional functionality, which basic routers don't have, such as intelligent layered security.
The main function of a firewall is to act as the first line of defense into your network, protecting it from external attacks. You can also use a firewall to open up ports for specific incoming and/or outgoing traffic. Firewalls also have logging facilities that will inform you about any failed attacks on your network.