Do you think that if you come up with a unique idea for an app, it's enough to develop it too? It is really not so. There is a lot of competition in the app's market. So, it is imperative to come up with a product that stands out, has a great UI, and being marketed at an optimum level.
As soon as the finalization of app's idea, people jump onto development directly without preparation. It could result in high development budget, lack of development quality, and ultimately a frustrating experience with the failed product. That is why follow these steps as soon as you have finalized your app's idea to come up with a fantastic app you were expecting.