Usually if I have an army moving around without a champion, it's headed off to join a champion whose army has been depleted, or it's headed off to garrison some point of the map I consider important (1-tile passes through mountains which bypass my main border defenses, or join the garrison of a border town or newly-conquered city, assuming I don't want to move a governor in and don't want to keep the conquering army there but do want to keep the city).
When at war with multiple AI factions and my empire is relatively exposed (say, I'm on a large open plain and don't control the edges of it), then I'll consider using armies without champions to help beat down the AI. I might also use such an army to suppress a wandering monster that I don't think is worth my time going after with a champion but do want removed from my territory, or to weaken particularly strong enemies (such as wandering dragons, or a major AI army that comes by just after one of my main armies has taken a beating but still has most or all of its units).
Mages and Commanders are useful enough as supports for my army, though, that I don't really want to fight a major battle without one, or sometimes both.
