Instead of relying on your credit card, you use the vendor (or someone else) credit, to pay over time. Looks like in some cases, they integrated in the vendor website, so when paying you can directly select Affirm. I did the test with Dyson, it redirected me to the Dyson website, and Dyson does already offers payment installments at 0% APR (12 months). Looks like Affirm does the math for you, if there is an APR, so you know the true cost. And it keeps all your debt nicely listed in the mobile app. So, you know what you owe every month. Hopefully Affirm helps you not overstretch... This could help people who do not fully grasp the true cost of those credits, and hopefully help them manage their debt.

Just currious, do you know if that help build personnal credit score ?

Good question... I would think Affirm does report to the 3 US agencies.