Its also used to purchase other cryptocurrencies. Trades ride the volatility of USD/BTC, as well as the XXX/BTC. In extreme cases you have USD/BTC, BTC/XXX, XXX/YYY!

While there is a large number of LTC, BTC and ETH exchanged for fiat (USD, EUR, RMB, KRW); Most of the trades are between BTC and alts - https://coinmarketcap.com/exchanges/volume/24-hour/