In android there are various storage options for users to persist application data. Normally android apps create and work with app data. Obviously critical data can be persisted in SQLite database or fetched via web services. However, temporary data can just be stored in the filesystem of the device. Android devices come with external storage that can be used to save and retrieve files.
Be aware that external storage can either be removable storage such as SD card or internal non-removable storage. Usually external storage is always a shared storage. This means the data there can be edited by the user or other applications. Suppose a user removes the external storage, it becomes unavailable to your app. It can also be unavailable when mounted to a computer via, say a USB cable.