Perhaps when your protagonists begin their journey to stop him, he calls up this army to oppose them based upon a prophecy--or what he thinks is a prophecy. For a bad guy to unleash that sort of army, he would need to be fearful that he'll lose his power in some way.
So either he unleashes them because of a prophecy, or for whatever other reason, he feels there's a terrible threat. Maybe the activities of the protagonists were exaggerated by one of his minions because the minion failed to hold them back. So rather than admitting, "Well a group of 5 people took out all of my munitions, a fifth of your army, and several supply wagons with some intelligently-placed bombs", he claimed that it was a HUUUUGE army that seemed to thunder down upon them out of nowhere, and then vanished... omg!
Either way, you want to focus less on what he would want it FOR, and more on why would he be afraid enough to summon it up. Whether to conquer or to protect himself doesn't matter all that much... something disturbed the status quo, and you need to decide what that was.
He COULD scare up that sort of army in order to go get some unobtainium from the neighboring city-state... but then you would have to explain why he couldn't do it with what he already has, or why he hasn't already done it.
So instead, focus on what scared him into believing he needed such a massive army, I'd say. He would be unlikely to suddenly become afraid of, or to suddenly become covetous of someone else's resources. Something has to trigger it, somewhere along the line.
Although, of course, you could go old-skool and just say he got bored. :p (Oops, someone already said that about being bored, lol)
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