There is absolutely no need to buy a third party app, you can fly with DJI Go App but...but there are few pluses in every third party apps. I'm I te rested in video aerial photography and wanted to plan well in advance how and where my P3 should fly at what sped and how the camera should be angled. It's difficult to do with DJI Go app and not possible yet to plan it offline. One of the apps that helped me is Litchi. I'm enjoying it. I could have picked up another app to do the same task but somehow I started with Litchi and staying with it for the purpose.
Know your interest and requirement first and then decide what to do, what to buy if any, can you take the risks etc...,,,so many question marks.