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You can use funds in your paypal balance 'as long as' they are in the correct currency and not pending.
BUT for some transactions ie digital / virtual or higher risk AND at 'some point' anyway Paypal will ask you to add a card to act as a back up funding source and to verify your Paypal account.
Like I wanted to buy a pack of GitHub stickers($5) for example, I clicked Checkout and then the PayPal button. A PayPal checkout page showed up asking me to enter my credit card information and address. I have $14 on my balance.