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I wouldn’t say that it is absolutely necessary to have both, both doing so adds another level of security. One of the main advantages of a hardware (router) firewall is that it is isolated from software on your PC. This becomes important if that PC becomes infected with malware, some of which can disable the software (PC) firewall. On the other hand, a hardware firewall is not very good at determining what application is trying to access your network, or whether that application is benign or malicious. On a home or small business network, you can think of it as the hardware firewall excels at keeping things out and software firewall excels at keeping things in.