iPhone Simulator is a crucial step in mobile app development as it allows developers to test their apps across multiple mobile browsers and devices efficiently. Manually testing each browser on each device is a slow process that can't keep up with agile development cycles, which require frequent releases. To speed up testing, automation using frameworks like Appium is necessary, allowing tests to run on hundreds of simulators, emulators, and real devices. This ensures the quality of the app before it reaches users, while Sauce takes care of device maintenance, security, and cleanup, enabling developers to focus on creating apps rather than managing infrastructure.