Specifically the two ends of the market; direct-to-consumer insurance (GEICO) and insuring things no-one else can, particularly reinsurance and muni debt (Berkshire Hathaway Assurance, Gen Re).

If you're good at writing insurance it's essentially a great way of getting interest-free loans which they then spin into a huge pile of low-risk, cash-generating businesses (e.g. Duracell, Fruit of the Loom, See's Candies, to pick three Berkshire brands...).