The idea is that you only control your hero. Without heroes, the battle between the smaller units is in balance. It's your job to tip the balance in your favour. The other type of characters (unplayable in this demo) are called generals, and they'll more directly influence the smaller units.
Not entirely true--generals won't really have much influence on creeps (although from what I've gleaned from the lua files, Vampire Lord will spawn his stuff by killing mobs, and maybe Oak will spawn some of his that way, too), beyond maybe Sedna healing 'em. They spawn their own units.
