Altcoin important if the altcoin bring something new like litecoin ( faster transactions ), dash ( anonymous ), or ethereum ( smart contract ). If it does not add any new feature or if the feature can be easily added to bitcoin network it means the altcoin is useless.
Beyond this there is difference between a single asset and multiple choice in the market. Just think of bitcoin, it'll continue to move on its path. The existence of altcoins give the bitcoin high competence