The purpose of implementing In-App Purchase is to add an in-app payment functionality to collect payment for enhanced functionality or additional content usable in your game. The following are options of incorporating this feature:
A game that offers new level packs to play outside the default content
A freemium game that allows you to purchase virtual currency to create or build new assets during game play
Adding additional characters or special power ups to enhance game elements
These are some examples that can be implemented with In-App Purchases.
In-App Purchases allow users to purchase additional content within an application. The App Store manages transaction information only. Developers cannot use the App Store to deliver content. So, either you bundle content with your app when you ship it (it will be unlocked on purchase), or you have to work out your own system to download the data, if you wish to deliver content.