I can give you more specific troubleshooting steps for that game.
It holds data required for "Zombie" or "MP" (Multiplayer) modes. fg-optional-mp-zombie-04.bin
If you try to launch zombies or multiplayer without this file, the game will likely close or throw an error indicating missing content. I can give you more specific troubleshooting steps
You do not need to download this file if you only intend to play the single-player campaign. typically for Call of Duty games
fg-optional-mp-zombie-04.bin is an used in FitGirl repacks, typically for Call of Duty games, that contains specific assets for multiplayer or zombie modes.