People are still playing with the crazy idea that the double spending problem has been solvedDo these coins try to solve a problem that one of the major ones dont(and by the way people are barely using the big ones in the real world)?
That in itself is why a lot of these coins exist...
And no most of them aren't useful at all, out of ~1500 cryptocurrencies listed on coinmarketcap, I'd wager probably 4 of them are 1) unique and 2) verifiably and reliably solve a real world problem. But that doesn't mean this won't change in the future. Cryptos are under constant construction so what may be useless today might not be the case tomorrow. It's all about adoption which will drive innovation which will drive further adoption. The trick is getting people on board.