Why do you use Linux instead of BSD? It's the same question.
Some potential reasons for using FreeBSD:
It's the same three firewalls forever, not a progression of different firewalls.
Old knowledge still works, for example, netstat is still the way to list socket connections and ifconfig is still the way to configure interfaces. I did some work with FreeBSD in 1999, and then nothing until 2004, and all the knowledge transferred, then in 2011 I changed jobs and skipped ahead several versions, and again all the knowledge transferred. But I kind of stopped using Linux heavily in 2013, and when I had to work on it at work in 2016, all the tools had changed, so I had to relearn (or just avoid the Linux part of my job, which I had the luxury of doing).
Official kernel support for ZFS.
Receive side scaling support is nice, if you need it; although I guess not a lot of people do.
Different positions on philisophical arguments like an integrated source repository with the kernel and the base userland software, or init systems.
Some potential reasons for using FreeBSD:
It's the same three firewalls forever, not a progression of different firewalls.
Old knowledge still works, for example, netstat is still the way to list socket connections and ifconfig is still the way to configure interfaces. I did some work with FreeBSD in 1999, and then nothing until 2004, and all the knowledge transferred, then in 2011 I changed jobs and skipped ahead several versions, and again all the knowledge transferred. But I kind of stopped using Linux heavily in 2013, and when I had to work on it at work in 2016, all the tools had changed, so I had to relearn (or just avoid the Linux part of my job, which I had the luxury of doing).
Official kernel support for ZFS.
Receive side scaling support is nice, if you need it; although I guess not a lot of people do.
Different positions on philisophical arguments like an integrated source repository with the kernel and the base userland software, or init systems.