Do you ever think that why are there so many currencies: USD, GBP or CNY...? The same question can pose here: are they needed if they're all about making payment? Besides making ease of payment, each altcoin serves a specific purpose: platform (ETH, ADA, NEO), cross chain (Wan, Atom) or something. That explains the diversification of altcoins. Many of them are trash, but many of them are born for some purposes.