If you have a house, it has a door which you can use to access everything inside.
If you have a linux install with no services running on it, it has no doors, and thus doesn't need any door locks. And if it does have services running on there (which run publicly), it now has doors, sure, but getting one of those doors open doesn't guarantee access to the whole house - usually it's gonna be an empty room
I think by services they mean self-hosted, web-based services, or things like sshd - services which work by actively serving connections on a particular port or ports.
And even then, a properly configured SSHD instance wouldn't really benefit from a firewall, unless you wanted to block all countries besides your own or something.
But… Is it?
It's not.
If you have a house, it has a door which you can use to access everything inside.
If you have a linux install with no services running on it, it has no doors, and thus doesn't need any door locks. And if it does have services running on there (which run publicly), it now has doors, sure, but getting one of those doors open doesn't guarantee access to the whole house - usually it's gonna be an empty room
Linux with no services? That's not a usable system for anyone who would ask "do I need a firewall"
Systemd is a service that runs logind. What are you doing without them?
I think by services they mean self-hosted, web-based services, or things like sshd - services which work by actively serving connections on a particular port or ports.
And even then, a properly configured SSHD instance wouldn't really benefit from a firewall, unless you wanted to block all countries besides your own or something.