If your router is setup to only allow in the ports with a service hanging off it, like SSH. Then a firewall wont add anything your router doesnt.

On the flip side, if your running any kind of VPS or directly accessible server, like a VPS or dedicated server. Then a firewall is required.

Now protecting your server from other things on your local network might something you want to do, think IoT stuff getting popped and being used to hack other things on the network