One of my favorite reasons is that the documentation is very complete, the OS is pretty secure and I feel there is alot more simplicity in using the FreeBSD operating system vs your favorite distros. Most operations you need to accomplish from file system navigation to building out IPSEC tunnels can be found inside the freebsd handbook. https://www.freebsd.org/doc/handbook/
The security offered by the system is rather complete. As mentioned before, jails are an amazing feature inherent to FreeBSD and the code sharing among the distributions allows some of the cool software developed by the OpenBSD team to get ported over. I am a huge fan of pf for firewalls, geli for full disk encryption and Mandatory Access Control.
If you are familiar with linux, the learning curve for BSD will not be very steep. One of the cool features about the DigitalOcean droplets is that your kernel is internal to the droplet. This allows you to easily perform tasks such as rebuilding your kernel or issuing a buildworld for OS upgrades or applying system patches.