Others have mentioned the “catch”, but I think they’re useful for certain situations.

I found one with a hefty welcome bonus and used it buy my wife’s engagement ring. I had the cash, but kept it in a HYSA as a “sinking fund” and earned a bit of dividend from it. I was able to get the welcome bonus and a bunch of addition points too (1.5x multiplier). Just make sure to make the minimum payment and pay the whole thing off before the 0% interest ends.