For security reasons, a router will block attempts to externally access a local network. However, in certain cases allowing external access can be beneficial. For example, this allows for you to provide others access to your network via Secure File Transfer Protocol (SFTP "Secure File Transfer Protocol"), and thereby set up a server to transfer your files. Everyone who would like to provide their friends with a small gaming server in their home network needs the IP address to do so. Minecraft and other games specifically offer game server software for this purpose.
To subvert the router's firewall, so called "ports" are enabled to allow external access. To dial in to the game server, gamers need the IP address of the server. The exact configuration of the firewall depends on both the operating system and on the game server or SFTP software.