Buffest uses his cash from his insurance companies as a super buffer, so that he can quickly purchase assets if wanted or needed. It also allows a company of that size to have very high market flexibility. Its actually not all that uncommon if you look through the history of the super rich. During the great depression the richest person in America, a woman whose name escapes me, used her considerable cash assets to vastly expand her holdings, and expand her wealth immensly.
As for the burger, for what around 100,000USD, you too can win a dinner with Warren Buffet.