The cavalry gain a charge bonus for each movement they make before attacking (as in speeding up before charging). That is why they are put in the rear: so when they charge, they do it much harder.
really? that's new to me as well, though I've been using cavalry forever
![Tongue :P](https://forum.battlemaster.org/Smileys/default/tongue.gif)
I thought they're put in the back simply to time their clash with the enemy infantry, so they charge at the same time as your own infantry.