I used to think in pretty much the same way before, but now I understand that yes, we do need many cryptocurrencies. The first reason for this is that some projects actually use cryptos for their projects anf cannot use the main coins for the same purposes. The second reasons is that even projects that need crypto only to accept it as payment are still justified in making their own tokens, because they can't trust top coins. It sound weird, I know, but look at how volatile the main coins are. They are mostly gaining their price from constant trading, while companies might prefer their own tokens with prices depending on how many exchanges of tokens for products occur.
Finally, more cryptos means less pressure on each separate blockchain, which contributes both to decentralization on the market and safe usage (no '$20 btc transactions' stuff).