Books have been written on the subject, so an HN comment isn't going to do it. But if you consider that anyone making a mutually consentual transaction is doing it because they value the traded good more than what they're trading, it's not hard to see how a free market can provide good things for people and society in general as resources flow to where they are most valued. Of course there's nuance here (such as assuming purely rational actors, addressing realites around information asymmetry, and that macroeconomics is different than micro) but that's part of the low-level foundation.

I'm not a big fan of capitalism as it can be an utterly cruel mistress, but given the unfortunate realities of human nature I can't think of a system that is on the whole better. As long as there is scarcity, there will be something used as a basis for rationing (who gets what, and why). If it's not who is willing to pay the most, then it's who has the most powerful friends or the biggest army, or something else. There are problems with under-checked capitalism, and there are problems with over-checked capitalism. We'll never get it just right because humans aren't smart enough to, but perhaps we'll achieve post-scarcity at some point. One can hope.