i use pink 32bit net-book with openbsd as a mostly command line computer, for when i feel like i wanna take a break of all the bloat of all my other computers. (all my other primary computer run arch tho...)
BSD is useful for when you want to program some specialty hardware once and chuck it into the abyss with no updates and without releasing the source code so you can maintain a barely passing level of security through obscurity. Or if you really just don't want to publish your source code for a unix system.
Things like modems and old routers come to mind. Also PlayStation iirc.
Most modern IoT stuff I've messed with uses linux probably because the devs like not having to manually package things and deal with weird edge case bugs. Since they're usually making software updates anyway, published vulns are less of a concern.
For the sake of her "they've hacked me" paranoia, my crazy sister made me install OpenBSD on her crappy PC three-four years ago (Intel i3 and a mechanical disk). She stopped using the PC altogether like 6 months after that. It wasn't really bad, everything seemed to work, taking in account the limitations of the hardware. The upgrade procedure irked me, though - mostly, realizing that you have to be reading documentation constantly even for a freaking minor version upgrade.
Still this made me try FreeBSD on my PC, only to realize after a couple days that pkg/pkgsrc are utter shit compared to Portage. Alas Gentoo/BSD is long gone, otherwise I'd love to try it.
The Mach kernel started as a derivative of a BSD kernel. Years later the XNU kernel was created by combining the Mach kernel with code from newer BSDs, therefore it's totally fair to describe macOS as a BSD. From my very limited exposure to BSD conferences, using Macs is pretty common there as many developers see their community-developed BSDs more for headless use they SSH onto.
I see little reason to use any of the BSDs. Neither for desktops nor for servers. The only benefit I see is that you can take the BSD licensed code and use it to create a closed source product like the PlayStation without having to contribute anything back. I dislike that benefit with quite some intensity.
I ran FreeBSD on my home server for a while since the old TrueNAS versions use it. The supposed simplicity of BSD rings hollow to me as it is just another thing I'd have to learn. I also don't care much about the Unix philosophy or any other clerical reasons that distinguish the various BSDs. Computers and their OSes are a tool to me not a religion. Admittedly TrueNAS worked well for me, but reading up on the differences from Linux got old rather quickly. I migrated to the newer Debian Linux based TrueNAS Scale a couple of months ago because I feel more confident that if anything goes wrong I'd be able to fix it.
openbsd seems interesting to me, it's entire existence seems to be "secure OS" and i think that's rather respectable. I'll get around to messing with it some day.
I used OpenBSD on servers for years. I don't think it's suited as a daily driver, especially not with a desktop. I absolutely love pf and miss it dearly, though. iptables and nftables are utter shit compared to the glory that is pf. Yes, there is some hyperbole in that statement, but only some.
I personally have tried FreeBSD and some FreeBSD "distros" on the desktop, and have used *BSD-based stuff as servers/single-purpose machines.
As a desktop system (user-centric use case), you notice how hardware support is sometimes problematic, especially on laptops. I personally had problems with NVIDIA GPUs, already a problem on Linux, being a big problem here as well, and don't mention WiFi (FreeBSD doesn't support 802.11ac and up currently) or Bluetooth. Software-wise, if your applications do not have a *BSD version, well, then you are relying on Linux ports, which for desktop use isn't exactly great.
But, in servers/headless setups, *BSDs are shining, with the most important things running rock-solid, stable and resource-friendly.