There's basically this whole category of things that function best as a monopoly. For insurance, you want the pool to be as large as possible to amortize risk

There's basically this whole category of things that function best as a monopoly. For insurance, you want the pool to be as large as possible to amortize risk over. For distribution networks, you don't want two networks covering the same area. When that happens, the most important thing is ensuring the monopoly is run well, for which strict regulations are a must. You must define a mission independent of profit seeking, requirements that must be met, and be able to hold people responsible if they fail to meet the requirements or succumb to excessive greed. After that it's largely irrelevant whether the entity providing the monopoly service is private or public.