In Europe, strong customer authentication is now required when making credit card purchases
https://en.wikipedia.org/wiki/Strong_customer_authenticationSo Klarna delays having to authenticate with the bank which issued your credit card from the time of purchase, to the time of paying your Klarna "credit card bill".
They also delay sending your invoice until the item is actually shipped, and you then have a 14 day due date. This means returns and refunds have no money exchanged from any of your own accounts, so you don't have to worry about how long it takes for the refund to enter your credit card.
Edit: They also remember you address based on your email and name, across any vendors, so check out is really quick even if it is the first time you shop at a specific site. This is of course great for smaller vendors.