Appium is a tool that allows developers to write a single test script for automation testing on both Android and iOS platforms. By using PageObject setup and annotations such as `@AndroidFindBy` and `@iOSFindBy`, developers can reference UI elements in their tests without duplicating code, making maintenance easier and reducing headaches. This approach enables developers to write a single script that runs on two different platforms, simplifying the automation testing process for Android and iOS applications.