A firewall monitors network traffic to and from the Internet as determined by configured rules.  Primarily it blocks threats coming from the Internet that may otherwise compromise the security of your network.  In addition to blocking threats from the Internet, most modern firewalls can provide UTM  functionality, Policy Base Routing , content filtering , load balancing  and WAN failover.   UTM and content filtering features can be utilized along with client antivirus to further protect your network from threats by looking more granularly at websites users are accessing, files users are downloading, and content that users are viewing to help prevent phishing attacks, Viruses, and lost productivity.  Load balancing and WAN failover refers to the ability to aggregate multiple connections to the Internet to increase throughput or utilize a secondary Internet connection in the event that one connection fails.