Another nice thing is that adding a unit changes the pseudorandom generator, which means you get a different outcome. So if you fight battle, you can re-run it a couple of different times to see if the outcome was close or pretty much inevitable, whether you were (un)lucky or using a good strategy. Just add a random archer or something somewhere and see if the battle outcome changes--repeat a few times. (Probably more useful in the late game with lots of units on the battlefield and lots of spells. In the early game, adding an extra unit can divert calvary formations and otherwise change the nature of the battle.)
-Max