When a device is deleted, the files associated with it and no other device are deleted as well. This is usually desired behavior, and in fact a common reason for deleting a device is to free up space in the account. It is not possible to restore a deleted device or its files, so before deleting anything you might want to review the details of exactly what files will be deleted. In general terms, that would be those files that both (1) originated with the device being deleted and also (2) are not associated with any other device.