Final answer:
Natural monopolies exist to ensure efficiency and reliability in markets with high fixed costs and low variable costs, leading to a situation where a single supplier is more cost-effective than multiple competitors.
Step-by-step explanation:
The main reason natural monopolies are formed is to ensure efficiency and reliability in the provision of certain goods and services that have high fixed costs and low variable costs. With the cost structure of a natural monopoly, one firm can supply the entire market demand at a lower cost than if there were multiple competing firms. This typically occurs in industries like utilities, where the infrastructure costs are high, and marginal costs of serving an additional customer are relatively low. Therefore, natural monopolies are often regulated to prevent the high prices and limited output that could occur if such a company were left unregulated.