These days, all but the smallest organizations spend mountains of money building redundancy into their infrastructures. As business depends more and more on those systems to function at even the most basic levels, the capital plowed into highly redundant disk arrays, bulletproof backups, and highly available virtualization infrastructures has become an expected cost of doing business.
However, the frenetic pace of break/fix, application rollouts, and systems upgrades often leads to the most dangerous single points of failure of all: people. That huge investment in redundant, self-healing infrastructure can be negated in one fell swoop if the one person who knows how to run some critical part of it quits, is on vacation, or even just went out for lunch without a cellphone at just the wrong time.
Often, you don't need an actual service disruption to cause a five-alarm political fire that reaches all the way to the executive suite. In my years as a consultant, I can't count the number of times I've been called in (sometimes at a substantial expense) to help fill a knowledge gap simply because a single member of the IT staff wasn't available for whatever reason.