You mentioned PLCs, so I supposed you are talking about some kind of OT network. A FW in those cases does not just help in the packet inspection (which is critical to check the content of the protocols), but in the definition of Operation Zones (Control, supervision, etc.), which are requirement for the compliance with many regulations in the industry field.
Many clients are deploying ruggedized/rugged firewalls in these environments just to help them defined zones, NATing networks (many industrial networks were isolated and the same addressing was used), and to perform DPI.
As I said, I would depend if you are trying to be compliant something, or in the deployment you already have, but a FW would help you in many aspects.