>>106290615 (OP)
you shouldn't, necessarily
there are advantages and disadvantages, as with every software choice
OpenBSD is reasonably secure by default, but can suffer from performance issues for some workloads, and you start chipping away at the "secure by default" business by installing third party packages
but if you can stick with their native httpd, OpenSMTPD, and OpenSSH, maybe that's a good choice
NetBSD can run on obscure architectures, which is probably irrelevant to your use case
FreeBSD has ZFS natively and supports heavy networking loads, but isn't nearly as secure by default as OpenBSD
if your question isn't just OS flamewar bait, describe your use case