It is good to use emulators in early stages of development. But when it comes to testing the real application must be tested on real devices. There are multiple factors like OS, Version, Memory, Hardware Specifications, Screen Size, etc which might affect the application behavior or performance that might not be seen on an Emulator. Lastly, the application will be used by real users on real devices and not on emulator, hence it is advisable to test mobile apps using different devices.