Yossarian is fixated on a certain event, so the story circles around that event until finally it gets revealed. It's a psychic wound that he keeps poking at, until finally he comes to grip with it. Then we see things that seem like throwaway jokes (like "Nately's Whore" or "Mais où sont les neigdens d’anten?") are core to the theme of the book.

The chapter where we get the full details of the event, "The Eternal City", is probably one of the most powerful chapters I've ever read in any book.