Paying your credit card off fully every month ensures you don’t pay any interest (use it like a debit card). Wrt the earning interest point, consider that with a credit card you still get interest on the money you don’t spend, and you can also collect interest on the money you do spend (eg I buy something for $100 on the 15th, I can collect interest on the $100 until I have to actually pay the bill on the 1st of next month). Not to mention that you also pay 1-1.5% less for everything you use a card for due to cashback schemes. I definitely see why it might seem weird the way Americans use credit cards but it is mostly rational.