It used to be (talking several years ago here) that Linux had lots of security issues. I don't know about FreeBSD so much, but we chose to use OpenBSD for all our network infrastructure (firewalls, filtering bridges, NAT boxes, etc), because of those issues. Now, as we upgrade those boxes we are converting them to Linux. Not saying that OpenBSD isn't still secure, but we are now confident that we can secure Linux, and we have an "installed base" of sysadmins who are familiar with Linux and the configuration of bridges and IPTables in Linux.

If you are wondering why someone you know chose to use FreeBSD, it could be they had sysadmins familiar with it, or were gaining a little bit of "security through obscurity" as Linux becomes more mainstream. Maybe someone else can come up with other reasons.

For me personally, when I have selected FreeBSD over Linux to solve a business problem it has been because of one killer feature: jails. With jails, it is trivial to run many, many server instances in a single host -- with minimal overhead. I realize this sort of pseudo-virtualization is possible with Linux as well, but with FreeBSD jails are an integral part of the base system.

Other considerations:

• If you have a host (e.g. a web server) that you want to be able to run indefinitely, FreeBSD is a good bet. It's possible to upgrade the entire base system, across both minor and major versions, without starting from scratch.
• For routers and perimeter firewalls, FreeBSD has some handy sysctl MIBs to tweak network behavior and a choice between two (three, actually) excellent firewalls: pf and ipfw.

My 5 cents.