What is the advantage of making purchases in-app as opposed to checkout (outside app)?

When I purchase in-game, the game takes me to the platform in which I am playing my game, i.e. Facebook, Google play, Amazon, Apple. Doing so, I can sometimes get a benefit, such as Google play credit or Amazon coins (no longer available). I hear that Apple users pay a little more. I have also bought through the Wooga webshop, which sometimes have better deals for my money.