The class rankings were something that existed back before the American Civil War, they officially began in 1818 (though the academy itself opened in 1802). It's usually just an interesting number putting someone at some point in a list, though there can be interesting results for some of them.

As to why rankings aren't referred to for other military academies, to the best of my knowledge (though my area of expertise is more the battles of the war), other military schools of the period in America didn't use a system akin to West Point's, though if someone knows better on that I'll happily let them field that part.

On the topic of the rankings themselves, a presentation in 2002 actually noted that it was usually those who scored in the middle who eventually rose to fairly high rank and performed best, like Stonewall Jackson, who came 17th in his class, U.S Grant who came in at 21st, or John Buford who came 16th in his.

Of course there were notable exceptions to this, George Custer, George Pickett and Henry Heth came dead last in their respective classes for example but rose considerably despite that, and Longstreet scraped in at 54th of 56. On the other end of the scale, Robert Lee, P.G.T. Beauregard and McClellan all came second their classes, and while McClellan was raised to high rank he was rather ineffectual as a commander (coincidentally the person who beat him also served in the ACW and topped out at Major in the Corps of Engineers). William T. Sherman came 6th in his class (though he would have come 4th if not for demerits) and Jesse Reno came 8th, so it's interesting how varied the results are, and how topping the class at West Point didn't necessarily mean you were the best of the best.