No, the only use case is not illegal activity.
NFTs, smart contracts in general, gaming, defi/lending/staking, DEX, DAO, Filecoin/IPFS, and a few more in itself are not illegal. You may consider them to be useless, but that's not the same thing as illegal.
Even if you scrap all that, there's the remaining core use case of speculation. Which in itself is also not illegal and an incredibly important use case, if not THE use case.
You may be frown upon speculation, look down on it, judge it anyway you please, but it does not change the fact that 100M+ people with an exponential growth rate are into it. I guess growing your money is popular, who would figure that.
The point I would like to get across is that instead of judging, you should have a deeper look at the WHY. Many people think that these risk takers are plain dumb, selfish, irresponsible. If only regulation would protect them against themselves.
That's not the situation at all. They're fully self-aware. They are young people born into an economy that is plain broken to them. Burdened by student debt, stagnant/unlivable wages, unaffordable housing, zero or negative interest rates, high inflation, no job security, unaffordable healthcare.
They barely get by and have no outlook of ever getting ahead, owning any asset or wealth, and the very simple goal of a basic middle class life has become unattainable.
In a backdrop that cruel, why not throw the little money you have into a shitcoin? It might do a 10x. It might also go to zero. Who cares? You didn't have any meaningful wealth to begin with.
And that is the appeal of crypto. It's not fueled by greed for the sake of greed, it's fueled by desperation. Nothing to lose, everything to win. Crypto is an asymmetrical bet, and the only one available to everyone.