emulators are not always recommended to use while developing the application. if you do not have the device with you then you can use it. for example, if you are developing the watch face for an application and you don't have the watch then you can use the emulators.

emulators are open source and provide an inexpensive solution. they can also provide unexpected behaviour of the app. you can run your app unmodified.

emulator consume lots of memory and will not be able to give real life expectancies and hence they may not provide real data about the way in which app will work on a real device.