If I ask you to write down the last 4 digits of your social security number, and then take you out to lunch and ask you how many dentists there are in Manhattan, there's going to be a high correlation between those two numbers. What happens is that the number psychologically makes you feel confident.
Failure saves lives. In the airline industry, every time a plane crashes the probability of the next crash is lowered by that. The Titanic saved lives because we're building bigger and bigger ships. So these people died, but we have effectively improved the safety of the system, and nothing failed in vain.