Inetd isn't a firewall. It controls a certain set of services which it starts up on an "as needed" basis. You can control which of these services it will actually allow to start. It is generally a good security practice to not allow services you don't need. For example, if you don't need telnet, then don't allow inetd to start it. On the other hand, iptables sets up your actual firewall. The firewall can actually block incoming packets. So say you need telnet for LAN connections but you don't want anybody from outside the LAN getting access to it. You can block those connection attempts at the firewall. Some services can also use tcpwrappers or some other method to control access. But the firewall can prevent the packets from getting to the service in the first place.
Hope this helps.