I used openbsd on a laptop some years back until that hardware died and my replacement didn't have some driver I wanted. It was a joy to use if you appreciate a light, no-nonsense Unix. Only pain point is that upgrades are very manual.
Now a decade later I have a different machine running freebsd which has a pretty similar feel, and I do appreciate the easier upgrades. I thought for a while in the recent past obsd had more up to date Intel graphics drivers but that was remedied in my use case by freebsd 11.
As for "why not linux", I feel like the BSDs as a desktop have a lot of the "on your own" feel that linux used to in the late 90s. Some people I suspect will loathe this. I like it.
Ditto that. Especially post- (dare I say it) systemd, Linux distros seem intent on going the commercial model of dictating how the "experience" should work, and I find I don't live in any of their target niches. Fedora is probably the closest, but is way too RHEL, and I kinda hate the way they do a lot of things.
The BSDs are by and for unix folks. If you're (for want of a better phrase) "culturally unix", they're a better fit.
But each to his/her own.
Debian on the same hardware was significantly more performant, so for me that answered the "why not Linux?" question
OpenBSD doesn't suffer the terrible churn and break-and-change-everything syndrome that makes the Linux UX so miserable. I can't compare it to FreeBSD, but I think OpenBSD can be more attractive for its focus on the head of development.
Why climb a mountain? Why cycle a 90km trail? Why travel the world? Because it's there and we can.