What's the point of z-scores?

I mean for stats analysis purposes. I'm faced with this issue. I'm correlating difference in exam score between two time points to the amount of study time engaged in between those two time points. At first glance, this isn't possible because you can't correlate a difference score to the amount of study time. I did anyway and the Pearson product is 0.67. A potential solution is to convert exam score and study time to z scores for correlation and/or regression. However, for obvious reasons, both linear regression and Pearson still come out at 0.67. In a case such as this I've seen it advised to convert the data to z scores but because there is no difference in outcome, what is the point?

Cheers,
DH