With APT and ZIP I think the way it works is you can set it up...

With APT and ZIP I think the way it works is you can set it up to automatically deduct payments when the payments are due. You don't have to manually go and pay them every installment.

I personally have enough liquid cash where I can just pay things in full then pay off the credit card too but not everyone is in the same boat as you and I mate.

The beauty of the system is you can use it at check out just like Paypal at the end of a shop, and then it'll deduct the payments. It helps with cash flow, especially if you are waiting on your next pay cheque and you want something special or there is a sale on. I Know this doesn't answer the question about why you would use it, you probably wouldn't and neither would I. But I see the advantages in it for a lot of other people and you should be able too as well if you take a step back. It's something I didn't realize when both these companies first launched.