It's to set system applications' resources aside, and it's more of an OpenBSD thing than a *BSD thing. Once in a blue moon you'll see it in the Linux and general *nix world as well:
_chrony:x:112: <- Debian _chrony:x:112: <- Debian _pflogd:*:18:
_rwhod:*:19:
_proxy:*:21:
_timedc:*:22:
_sdpd:*:23:
_httpd:*:24:
_mdnsd:*:25:
_tests:*:26:
_tcpdump:*:27:
_tss:*:28:
_gpio:*:29:
_rtadvd:*:30:
_unbound:*:32:
_nsd:*:33:
_dhcpcd:*:35:"When maintainers choose a new hardcoded or dynamically generated username for packages to use, they should start this username with an underscore."
https://www.debian.org/doc/debian-policy/upgrading-checklist...