It does much more than simply light the cross hair, or rather the chevron, donut, or horseshoe reticle on some models. It provides what is called the "bindon aiming concept". It's hard to explain, and it doesn't work for everyone, but it almost lets you be as fast as a red dot up close. I guess the best way to explain it is; when you are moving the scope, and the image in it is blurred, all your mind sees is the illuminated reticle, but when it stops your eyes/mind focuses on the now clear image. I used one for years, and still do bust it out when shooting further than 200 meters or so, but I now mostly use an aimpoint, mainly because it makes holdover so much easier up close. With the magnification on an ACOG it's slower because you have to think more about how much higher you have to aim.
It is freaking bomb proof though, I've dropped entire other rifles on mine, and it didn't lose zero, and the glass is very nice.