Since it's easier to understand something by comparing examples of good and bad, let's compare two hitters with good and bad Posture.
In this case, I've chosen Don Mattingly and Joc Pederson, in part because what Don Mattingly teaches is different than what he did during his playing days.
One obvious difference between Don Mattingly and Joc Pederson can be seen by comparing their Posture both initially and at the release point; from set-up to release point, Don Mattingly gets into his back leg while Joc Pederson doesn't.
A more important difference is that, relative to the pitcher's delivery, Don Mattingly gets into an athletic position sooner than Joc Pederson does.
Don Mattingly vs. Joc Pederson
And why does that matter?
Compare Don Mattingly's head movement from Frame 2 to Frame 3 versus Joc Pederson's head movement from Frame B to Frame C. This moment is critical because it is the main time the hitter has to read to pitch.
Notice how, during this time, Don Mattingly's head is far more stable than Joc Peterson's.
What's more, Don Mattingly also exhibits less head movement from Frame 3 to Frame 4 than Joc Pederson does from Frame C to Frame D. This moment is less critical, because during this time the hitter has likely started his swing, but Joc Pederson's greater head movement gives him less of a chance of picking up and making a last-second adjustment to late movement of the pitch.
In sum, my concern with Joc Pederson is his major head movement after the release of the ball as opposed to Don Mattingly who adjusts his Posture sooner and then mostly just rotates.
Don Mattingly adjusts his Posture INTO the release of the pitch while Joc Pederson adjusts his Posture AFTER the release of the pitch.
A batter's Set-up and Stance doesn't absolutely matter until release point. It is just style til then, unless you hold a bad stance for too long.
Stance is style for Mattingly but substance for Pederson.