The BlackSwanTheory is used by NassimNicholasTaleb (in his book the TheBlackSwanBook) to explain the existence and occurrence of high-impact, hard-to-predict, and rare events that are beyond the realm of normal expectations.

Like, for example, the day a software project is going to end... or how much it is going to cost... or if it is going to be wildly successfull... or a catastrophic failure that will cost the world millions in damages...