I frequently use a credit card through PayPal to buy online. It is much safer than giving your credit card number directly to a merchant. PayPal adds a huge extra layer of protection to using a credit card online.
I do not transfer money from my bank account to my PayPal account, so my PayPal account is usually at 0. If I want to buy something online, my first choice is always to use one of the credit cards I have added to my PayPal account (these can be ANY credit cards, not necessarily PayPal's credit card, and you can add as many as you want) because (1) I get the extra insurance of paying with a credit card, such as being able to dispute a charge, (2) the credit card I use has reward points I can cash in later, and (3) doing the cc through PayPal is safer than just using a cc.
If I can use PayPal but not a credit card (for example, if I want to buy something from someone listing on the QB thrift shop area), then I pay with the default option of PayPal. It is an instant transfer because I have linked my bank account to my PayPal acccount and have also linked a backup credit card.