There are several, to be honest, but my favourite by far is that it cleanly separates, like all BSDs, the base system and the external packages. You can have a stable or bleeding edge base system depending on what suits your tastes, and on it install the lastest extra packages you need. They completely reside under /usr/local and are installed through different channels (the ports), cleanly separating the system from the user software, like it happens on Windows or macOS but arguably less clunky.

This is something is sincerely miss in Linux, where more often than not you have to either decide between having bleeding edge packages on a bleeding edge system (like Linux, or Debian Sid) or having to deal with old packages and third party repositories (like on Debian or Red Hat).

Also, the fact the whole system is shipped, tested and built by a closely knit group of developers is a huge bonus to me - the overall experience feels polished and well integrated, something Linux distros often seriously lack. At times on Linux it just feels as if some packages have just been bundled haphazardly together with zero care about documenting their interactions or how they are supposed to work, or that the packagers do not agree with upstream and do random modifications that just complicate things (like on Debian or Red Hat). Lots of stuff in FreeBSD has no upstream - FreeBSD and the other BSDs write and maintain their own kernel, libc, core utils, init system, bootloader, ...