I would say the biggest advantage is that OpenBSD is a very security-focused distribution, in a way that I don’t think any Linux-based distro has adopted.
The other advantage is ZFS. 10-20 years ago, there was no equivalent, and btrfs was in its infancy. These days, btrfs has proven that it is pretty stable and resilient. There might still be some advantages of ZFS over btrfs, but I haven’t used either one at all, so I can’t really be sure.
Outside of that, the BSDs are basically just different distros. Back in the 90s, when there was a lot more diversity in Unix, a lot of people just started out with *BSD because there was no clear choice at the time. People just like to use what they are more comfortable with - but most new users pick Linux over BSD these days, and a lot of people who started out on BSD have assimilated onto Linux.
Still, diversity is a good, nice thing, especially with the advent of systemd. So I’m glad we still have the BSDs around, even if I disagree with their stance toward the GPL.