While making their own missions for Zombicide: Chronicles allows GMs to explore the post- apocalyptic world in their own way, having pre-made missions can really make life easier. They can be used as-is, or tailored to any players campaign, used as one-shot adventures during a vacation or convention, or whenever a GM doesnt have time to create a full session themselves.
The Mission Compendium for Zombicide: Chronicles gives GMs 10 additional missions that they can use in their games. They can be added to a pre-existing campaign, or used whenever the GM needs them.