You can be asked to find an observed value as in the following question.

A shop sells a particular T.V at a mean rate of 5 per day. Assuming that the number sold per day follows a Poisson distribution, find the smallest number of T.V.’s that must be in stock at the beginning of the day in order to have at least a 99% chance of being able to meet all demands that day.

Poisson Distribution - Calculating an observed value from tables | ExamSolutions - youtube Video