The other commenter is comparing FSA to HSA which is right I think. I think FSAs work for some people (I never understood who though) but there’s literally no downside to an HSA. It basically can end up as another tax sheltered investment account, if you have enough money/luck to be able to pay off your healthcare costs out of pocket.

Like everything in the US, it’s amazing for people with money. Less useful for those that don’t. But at the very least it provides a buffer for the insane deductibles that US persons need to pay to keep living.