The empirical rule says about 68% of a normal distribution lies within one standard deviation and 95% within two. Why is this worth memorising rather than looking up?

D. Because it converts a measurement into a judgement of how unusual

It turns a raw number into surprise on sight: one SD is ordinary, two is notable, three is rare. The caution is in the premise, it holds for roughly normal data, and applying it to skewed data like income will badly understate how many people sit far above the mean.