I think you can summarize "what is the government good for" with "solving collective action problems", but libertarians tend to think that there are not many important such problems because market solutions are so versatile.

...but the most important collective action problem IMO (ever since someone told me about Ronald Coase) is maintaining low transaction costs in markets so they can function. So free markets and (good) government seem to me symbiotic.

https://en.wikipedia.org/wiki/Collective_action_problem