Let’s say you have developed an application on iOS and want to port it to Android.For sure, the best situation could be achieved in case if your app has been developed on the cross-platforming tool, which allows a simultaneous development of an application for several platforms. There are a lot of such softwares and fundamental extensions, that may help you in achieving such goals, for example, Unity, where you can do any operations for any type of UI (User Interface) and level of complexity. The code for iOS and Android app is pretty much the same (the difference contains in 2 to 5 percents of the code), so you can write only one time and use it for any platform – iOS, Android, Windows, Mac OS, Linux or etc. But that is a utopia, so let’s concern more realistic situations in porting the iOS app to Android. and vice-versa. The first thing you start with is a realization that you need to build a whole new application from scratch (without a back-end though)

A few reasons “for” porting to Android (or vice versa):

It has your back covered. Your competitors will not be able to steal 85% of the user market right in front of your nose – as that’s the percentage of Android-owning customers.
More apps simply mean more revenue.
Make money with Google Play
Reduce app development costs