I use it to keep a general idea on how useful certain units are in my army, and to generate a general outcome of certain pair-ups in a given situation. Such as when I was working on a dual Redeemer list, I ran the numbers to see how "killy" the set-up was that chose, then ran those numbers against a few different Opfor armies at the same point total, just to see if in was worth the effort, and finally play tested in to see how it actually ran in real games. In the end, math-hammer gave me the impression that the set up was a bit mediocre, while the real games should me the set-up was situational at best. That's math-hammer at work!
SJ