Because I don't want to give the credit card details to every site out there. And Because the Resolution Center works wanders with merchants who are not being forthcoming to resolve your problems. I once had an issue that a merchant had delivered less than half of the items that I had ordered, i contacted them and they requested (after 2 days) Proof that I had not received the items. I could only produce the photo of the opened package which was clearly too small to contain everything they were supposed to deliver and the weight in the package label that clearly was too little for everything I was supposed to get. They tried to stall asking proof that i had not received a second package with the rest of the missing items.(How can you prove a negative?

I got fed up and opened a refund ticket with paypal describing the problems and within 30 min the merchant contacted me promising to send the missing items and refund 20% of the cost if i closed the ticket in paypal.