Do I Need a Firewall If My Software Is Up-To-Date?

A firewall is needed whether or not you are extremely vigilant in keeping your operating system and other software up-to-date with patches created by the manufacturers.

One reason is that vulnerabilities in software can be found that take days (months) (years??) to be patched. Sometimes, these vulnerabilities can be exploited as soon as they are found (a 0-day exploit), with information about the exploit being sent around the Internet to those with malicious intent

Rather than waiting for an exploit patch and hoping that your computer does not get attacked, firewalls can help. Firewall manufacturers may be quicker than software manufactures to issue updates to block traffic that can trigger exploits. However, since firewalls already block a good deal of suspicious traffic, a firewall user may already be somewhat protected against new vulnerabilities without having to do anything else.