freebsd = utility: performance, gnu software compatibility, drivers, closest thing to useful as a desktop (still not very), but almost x86 only
openbsd = security: internet exposed services
netbsd = portability: already ported / most easily portable to every cpu arch & platform including old and new obscure ones. If you want a big-endian os for whatever academic reason, it's netbsd on an old sun sparc or pi3. I had an HP Jornada that had a SH3 cpu and shipped with wince, the only alternative os on that was netbsd.
NetBSD runs beautifully (but not quickly) on a 32-bit Raspberry Pi Zero. I hosted a mail server on that platform for about 7 years. I absolutely love how spare the system is when you start it up. You don’t even get a fancy shell, just sh. Running top on a new system is hilarious. It’s like init, your shell, and something else.