I think alot of trader prefer to trade altcoin on pair stable coin than BTC, because will easy to determine P/L and its more easy to do TA. When I started learn how to trade crypto in 2015 alot of altcoin paired with BTC and make me hard how actually the price in USD but now alot of altcoin paired with stable coin so make more easy to trade, especially for beginner trader. Stable coin needed not only for trading but also for yield farming, staking, diversification risk and others purpose.