Earlier quoted context omitted.
0% APR over 12 months for a big ticket purchase like a mattress in an inflationary environment and a bull market can easily equal an additional 5-7% discount on 2000 dollars. Admittedly this is not how most people are using Affirm, but it's definitely been useful to me in that way.
If you pay your balance in full each month every card is 0% forever, at least in the U.K.
It is very common to get a credit limit in the range of $10-20k on these cards, one could easily put all expenses on a 0% interest card, make minimum payments and put the amount of the statement balance in a high yield savings account until the promotional interest ends.
In total one would get negative interest in the amount of the rewards (say 1.25% average) plus the 15+ month yield on the cash stored away.