Question regarding nagios user


I followed the installation guide and created a “nagios” user. When I restart nagios, I do so as the root user.
With that said, where does the “nagios” user come into play? I ask because there’s no reason for me to allow the nagios user to have a login.
I tried modifying the default shell to /sbin/nologin but starting nagios up complained that the user doesn’t exist. It runs fine but I wasn’t sure
if I’m doing it right.