A legitimate practical use is anonymised online transactions using cryptocurrencies designed for such purposes (like Monero)
Might as well use reddit if you cant see a little (if not a lot) of benefit to using something decentralized over its centralized counterpart.
The anonymous aspect and lack of fees/operation should be self explanatory.
The problem is that is used for illegal shit moslty. Be it the aformentioned or scams. And the problem is that money as a concept is centraliced and controled by whatever entity holds the most of whatever is based on, be it gold or whatever. It shouldnt but it is. And the whole " you better use reddit since you suck centraliced feet" thing, to that i say to you grow up.