If you imagine playing a video game and you want to buy something that is in that videogame.
You can't imagine a “product detail page” or a rectangular 2D page with a scroll in a 3D video game, can you?
That's where commerce APIs like CommerceLayer come in handy.
Why?
Because you can attached an "add to cart" button to anything, you don't even need to add the "add to cart" button, you just call the add to cart API function and you can add any object to you cart by clicking or tapping on the object itself.
Easy enough isn't it?
Well yes! If you consider that you may need to pay for the object and choose a size if necessary. But a part from that you can make anything shoppable.