In the real world, we often find ourselves asking questions pertaining to "counts": how many car accidents in a day? How many potholes in the road? How many buses in an hour? How many defects in a square meter of product? When these events are random but follow some kind of pattern, we can use probability to predict the answers.

The probability distributions we know thus far can't model any of these situations. If we want to find a way to describe counts of random events, we'll need to introduce a new distribution for random variables, the Poisson distribution.