It took me an awfully long time to figure this out. Maybe longer than most who've duly applied themselves to the question. But here is what I've concluded:
This world is not a boring place. Not in itself. But it can all too easily seem boring when we look at it - as we often look at other people - solely for what we can get out of it. When we see it as something created primarily for ourselves; or worse yet, for those we most admire or want to be like. All those "cool and smart people," etc. Or, worst of all, when we see the world as something that chiefly we ourselves, and other cool people like us, have created. With little or no input even from God.
But that's not the world's fault. Indeed the Creation itself is so cram-packed with so many things full of wonder, and absorbing interest, and sometimes even delight, that even its dullest creatures - those things that seem to us most merely "what they are," or "what they do", and nothing else - even these can surprise us. Even these can somehow manage to exceed and escape our attempts to functionalize them: to make them dull and predictable and serviceable. Or so they may, at least, when we push them too far? - too far beyond their natures, and the laws and limits of their natures? Anything in nature - and that includes our human nature - anything in our physical world when pushed too far can spring, or snap back. With the consequences becoming at times not just inconvenient, but violent or even ugly. Like, for instance, the Great American Dust Bowl of the 1930s. Or even (some might argue) the Irish Potato Famine of the 1840s. But also, just conceivably?, in ways that a certain perennially popular genre of science fiction - everything from Hitchcock's The Birds to The Walking Dead? - never tires of reminding us.
In sum, there is no creature in this universe so merely useful as to be reducible to its mere utility. Not even birds. But least of all ourselves, and each other. Indeed, it is just those things in you and me which are most "irreducible" - which are most indelibly "ourselves," as it were (and yet often seem most useless) - that are the seedbed of everything good that lies "beyond" them. I.e., it is precisely these elements - most often encompassed under the terms "soul" or "spirit" - that, when heedfully discerned, cultivated, encouraged, can become the groundwork of all those other, "practical" things about us, that make the world sit up and take notice. Which is to say, all those skills and talents that make us useful in the best, completest sense of the word, because they make even busy, important, sensible people actually need us, and depend on us. As opposed to regarding us as so much raw material that may be infinitely serviceable, but is also infinitely downgradeable and disposable. The lesson here is that every human soul is not just a thing beyond price; it is, as designed by its Maker, far too efficient a thing for any of us to presume we can afford to dispense with it. Or use it up. Or worst of all, like Mick Jagger's Lucifer, "lay [it] to waste."
My point is that no God-designed creature is so dysfunctional as to be either without use, or else nothing more than what can be reduced to its various uses. So, yes, even the Devil, that great waster, has his uses. And yet even he - so far as he is still God's creature and not his own - remains more than any of them. (Which also implies that the converse is true: To the extent that he has become "his own god," he is not just "reducible" to his uses, but is much less than any of them. But more on that in my final paragraph.)
So what might be one utility* of Satan for the patient, heedful Christian?
*If that is not too presumptuous a word for One whose cosmic authority and eminence remain very real.
He is, may I suggest, a reminder to us that there are, in this universe, creatures of super-extraordinary intelligence, and rationality, and insight, not to mention power - power to calculate and predict and persuade, to dominate and control. Creatures so supreme, in a word, that if we could see them as their Maker first intended them to be, we might easily be tempted to regard them as no creatures at all, but rather as some species of deity. Even to the point of bowing down and worshiping them, and yielding our lives to them. What prevents us, by God's grace, is when we see the fallen angels clearly for what they have since become - or rather have made themselves - in the naked glare of Divine judgment: beings utterly hateful, both to themselves and to us.
Yet here is a paradox. What makes demons hateful to us is not that they intended to be so, at least not at the outset. No matter how much they hate us, they had every intention of being esteemed and worshiped and adored, even by humans. Rather are they an example of what can happen to any created being, even the highest seraph, when it becomes obsessed with how it can use, with what it can make of any thing - even itself - to the neglect of what that creature merely is. Because what any thing "merely is" *(as distinct even from what it thinks of itself) is always more inexhaustibly interesting - more pregnant with symbol and mystery, with story and meaning and love beyond its own powers - than anything you or I could ever make of it.
So why do we often behave as though matters were just the opposite? Why do we too often presume, whether consciously or otherwise, that things - and even people - only become more interesting in the degree that we "monkey with" or manipulate them?
The problem is that seeing what any thing "merely is" is no easy task. To see any creature for itself - both its inestimable value and its innumerable uses - presupposes something very different from the kinds of human company we're most used to in a busy, hands-on, workaday world. What is required, rather, is a presence so lowly, and quiet, and "ear-to-the-ground", so preternaturally heedful and hands-off, that it can only be Divine. In other words, to see any thing clearly demands much more than any such talent, or ingenuity, as even the most gifted fallen angel could supply. Even more than an active, probing, dissecting intellect, it requires that almost animally patient, yielding, trustful, and teachable thing we call wisdom.
With Lucifer, then, may I suggest that we have a kind of ultimate test case, or object lesson, in what happens when any created thing becomes so supremely, commandingly intelligent, and so conscious of its intelligence, as to leave no room and no use for wisdom. But in particular, I think, for the sort of "omni-sensitive," omni-appreciative wisdom which is closest to the heart of God. And that is best secured by, if not inseparable from, humility.
My point is that even the most supremely gifted seraph can reach a dead end. And he never does so more totally than when he becomes so obsessed with the use and the self-making of anything - including himself - as to make himself all but totally useless.
(And once again, pray for the peace of Kyiv. And in particular, pray and fast for the pending peace talks between Trump and Putin. Granted, you may hate them both absolutely. But ask yourself: Are you absolutely sure you can afford to have things to get even worse?)