Data drawn from a normanormal distribution assures that the z-score has a normal distribution. The central limit theorem only says that the z-score converges to normality, and it doesn’t even say how fast, so our 30 samples might not result in a very normal-like z-score (though the convergence is often quite fast...while it’s just a joke, there’s a reason that I say that statisticians think $30=\infty$).