With just NAT someone who is on same L2 domain as you could you packet that has private IP address in the destination address. If there's no firewall (i.e. no deny rules for packets coming from outside interface directed to private addresses or more commonly just blanket deny for all non-established/related packets), router would happily forward the packet. The return packet may end up being weird (router may or may not change the source IP & port), but the attacker would be able to talk to the given host.

Firewalls may not even have NAT functionality. They are used to figure decide if packet coming from interface with some flags (source, destination, ip, protocol, port, tcp flags etc.) is allowed to be passed to another interface. These days virtually all routers have at least basic firewall support & firewalls have at least basic NAT support.