Why would someone choose FreeBSD over Linux?
If you want to know what's different so you can use the system more efficiently, here is a commonly referenced introduction to BSD to people coming from a Linux background.
If you want more of the historical context for this decision, I'll just take a guess as to why they chose FreeBSD. Around the time of the first dot-com bubble, FreeBSD 4 was extremely popular with ISPs. This may or may not have been related to the addition of kqueue. The Wikipedia page describes the feelings for FreeBSD 4 thusly: "…widely regarded as one of the most stable and high performance operating systems of the whole Unix lineage." FreeBSD in particular has added other features over time which would appeal to hosting providers, such as jail and ZFS support.
Personally, I really like the BSD systems because they just feel like they fit together better than most Linux distros I've used. Also, the documentation provided directly in the various handbooks, etc. is outstanding. If you're going to be using FreeBSD, I highly recommend the FreeBSD Handbook.
FreeBSD has a reputation for a more robust network stack. From professional experience at a previous company, we had a proxy server that was falling over from the load. When we threw FreeBSD on it, the server handled the load with ease for well over a year (I moved on - could still be working).
NetBSD has a reputation for running on a ton of different hardware.
OpenBSD has a (well-deserved) reputation for being extremely secure.
It's Unix, it's robust and it's free. No real reason to avoid it, but you'll miss out on the new hotness that the Linuxes tend to have.
I prefer the license philosophy of BSD license vs GPL license. To me, free means do pretty much whatever you want with the code. It's so free you can make it not free like apple did.
Practically it probably has no impact on me, but I prefer it on principle and was one of the reasons I chose to use FreeBSD over Linux.
Another reason is I wanted to tinker, I find when when I use Ubuntu I'm not putting on my unix hat; instead I'm just using the GUI everywhere as if I was in windows (which is not necessarily a bad thing just different).