The credit card serves as a back up if there is no money in the back accound that is connected to your Paypal.
When you pay through Paypal (instant transfer from back account) with no credit card, the payment is blocked for 7-10 days before it gets cleared.
When your Paypal account is backed with credit card, any instant transfer from back account is instantly cleared and the seller can immediatly send the item to you.
I'm using eBay/Paypal since 2008, you can trust Paypal