What is the inverse norm. calculation used for? Is it just to calculate the z value from a given proportion?
Yes. If you want to know the z-score for which a given percent of data lies to the left, you would use invNorm. On a TI-84 you can additionally tell it what the mean and standard deviation of the distribution are and it will convert the z-score to an actual value.
invNorm(leftArea, mean, SD)
Some later versions allow you to specify a right tail area or a central area.
It helps you to turn percentiles into Z scores