Servers operate over networks and are accessed by multiple users / devices both locally and from the Internet. Adequate security is required to prevent data loss and service disruption from network threats.
Why pay for commercial security software and outsourced IT support when virtualisation allows you to create an open source security server to act as the network gateway providing firewall, web filtering and secure remote VPN access just as secure if not moreso with more powerful resources and is absolutely FREE. Yes, open source security requires greater skillset and years of experience to administer, but isn't that what you are paying for outsourced support for?