Since your home router blocks all incoming ports by default, and your phone still works, you can still use a firewall, but I don't see an actual reason for that.
The truth is, there is no need for a firewall for Android devices as long as you use reputable Android apps from the Google store. You are unlikely to encounter any issues as long as you are not using any suspicious apps which leak information (even reputable apps from the Play Store may leak private information to third parties for the purpose of advertising, and that one may not like it).
Even though you installed a firewall on your device, you are not fully protected against WiFi eavesdroppers in cafes, coffee shops, airports, schools, and hotels. In such a case, you can secure your wireless internet connection using a VPN connection to create several security layers. When connected to a VPN, all your Internet traffic is sent from your computer/device through an encrypted tunnel to the VPN provider’s network. Thus it’s encrypted and secured from any local Wi-Fi eavesdroppers at the hotspot.