I guess, the idea was to put a group of your units under AI control so you can concentrate on issuing commands to key units. Nonetheless, the result was, as someone here suggested, AI as in "ARTIFICIAL IDIOCY"... Units put under AI control behave in a completely idiotic fashion: even worse than the real AI units (i've had them break formations in "guard mode" and start pursuing some lone enemy unit across half the map... or, i'd put 4 hastati engaged into combat in a town square, basically, eliminating the surrounded defenders, under AI control, thinking it should be able to handle THIS... the 4 hastati would just stop fighting and start walking towards the gates......)... so far, i've seen no use for this feature, really...