With Klarna you just need to type some information most people know by heart (10 digit ID number, f.ex.) before the order is confirmed. This is convenient if one wants to buy something quickly from a mobile phone. The address will oftenalso be prefilled. A credit card number is much more cumbersome to type on a small device, and the address needs to be typed in separately.
Some banks used to require people to log into their banks to temporarily unlock their card for internet shopping, or, nowadays, one also needs to authenticate the purchase with the bank. That adds extra friction.
With Klarna one does not need to pay until 14 days after the goods are shipped. Credit cards are even better, but most people tend to just have a debit card. With Klarna they don't need to worry about spending too much money from the account and having some other payment bounce later on.
I personally stopped using them after I fell for one of their dark patterns and bought something on credit, which incurred an extra fee. Legally I was entitled to cancel the credit purchase, pay the full amount and avoid the fee; but I was still annoyed.