Whereas one job of a personal firewall is to block potentially malicious inbound connections to your machine, another is to block potentially malicious outbound connections. For example, if some malware does find its way onto your system and then it attempts to "phone home" with whatever sensitive data it may have found, a good personal firewall should stop stop most outbound communications dead in their tracks until the end-user explicitly allows it (one problem with such conditional blocking is that end-users are rarely presented with enough information on which to base a decision).# Read on at ZdNet