It depends what the spirits can do, obviously. But if he already rules the world, then why indeed?
Perhaps he lost a loved one or has some obsession with death, so he wants to merge the worlds of dead and living. Assuming these spirits are dead, of course.
Or maybe he is concerned about some particular threat to him, which his current resources cannot cope with, so he's recruiting these spirits as additional reinforcements. For instance, maybe he's got a spirit enemy that requires a spirit army to defeat. (If you want to toy with the moral axis, let him unleash the evil spirits to defeat an even MORE evil thing, which threatens his kingdom and his people...)
Or maybe now that he rules the world, he's recruiting the spirits to help him conquer a NEW world, which only they can enter. (He might be going extradimensional, or storming the gates of Hell, or attacking the gods in their home plane, or whatever.)
Or maybe he's bored of ruling the world, and now seeks to destroy it.
I've seen a number of plots that involved Boss Villain releasing some ancient terror(s) from its can, but typically they're doing it in a bid to get power they don't already have. There's a handful of nihilists like the Joker, who just wreck whatever they can because they think it's fun... but it's hard to do that kind of villain convincingly.
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