Well maybe these coins are similar in their use cases and features so you could ask why do we need them both, but if you do that in that case you also need to do this with basically every industry and their products in an open market.
Why do we need cars from BMW if we already have Mercedes cars? Or why do we need Smartphones from Apple if there already is Samsung.
The answer is because some people like BMW more than Mercedes and Apple more than Samsung for completely individual reasons.
And the same thing applies to crypto projects too, because there aren't much really unique projects out there which offer something that all the other projects cant offer in a similar way.