If you try to move that save file to another Xbox, the console checks the signature. If the IDs don't match, the console rejects the file—displaying the infamous "Corrupted Save" error. The bypasses this by decrypting the save, extracting the raw data, and re-encrypting it with a different set of credentials.
Note on Modern Consoles: When the Xbox One launched, Microsoft shifted to a more robust AES-256 encryption standard. Traditional GSX Resigners do not work on Xbox Series X/S native titles. However, backward-compatible Xbox 360 titles running on Xbox One still utilize the old container format, meaning GSX Resigners remain relevant. gsx resigner