It's my nameserver/DHCP server. I used to have it set up as an iSCSI target for backups and as a boot server for my firewall, but I do something else these days.

My main reason for using NetBSD for this is to have easy access to the man pages. Like the other BSDs, the man pages are exceptionally well-written and are a tremendous resource for doing POSIX programming. Plus I find myself digging through the code when I'm interested in how something is implemented. Having a local repository of good C code with a liberal license is worth having the extra OS to manage.

I use it on a couple of old laptops that Linux no longer supports.

It was my desktop daily driver circa year 2000.

Not sure what people use it for now.

i ran NetBSD as a desktop on BeagleBoneBlack from circa 2010 until recently when I donated the hardware to a CompSci student.

...and as others mentioned it: the rump kernel.