Bitcoin is the world's first electronic currency and it is also used for exchange for altcoin. So if everyone wants to buy Altcoin, then everyone must buy BTC, then you can exchange Altcoin that you want.
but i think that it is not necessary for every currency, a lot of exchanges are there who allow you to convert your crypto currencies in to USD directly and you will not feel the need to convert your crypto currencies in to bitcoin, but still i think that we cannot ignore the importance of bitcoin, bitcoin keep too much importance in our life.