What is the purpose of the “developer payload”? What can it be used for?

Question

The Android docs say that its meant for "supplemental information about an order" but at the same time it also says not to use this to send "actual data or content".

So what is the purpose of this "developer payload"? Why does this feature exist? Can you describe a real-world practical example of how I can use it in my own In-app Billing implementation?