Yes, because you can forget what services are running and maybe they can be explited.
An example can be Syncthing which setting are done via web browser at port 8384. If you do not have a firewall, everyone on the same network would be able to change Syncthing settings and then sync your directories to their devices.
Good point, but syncthing only listens on localhost as others have said too.
However it still is a network service that can have vulnerabilities, besides many others like KDE Connect (which may be a target as one of its purposes is remote control and monitoring) or a bittorrent client