Why do i need to add one debit or credit card to my paypal account?

[rogerblanco]

Hi, I just sold one item in ebay and I accepted to be paid in my paypal account before to sell it.,

But, If I want to buy something in GNC.com and go to: check out with paypal account, later, they will ask me for my debit or credit account.

The question is: If I have money in my paypal account, why do i need add my bebit or credit cad number?