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.
It depends on how you are calculating your profit and loss. If there is no stable coin then traders would be calculated profit and loss in satoshi. what I think is a stable coin helps us to recover our losses more easily and save our portfolio in market fluctuations. Actually, it mainly helps to secure our funds in highly volatile market conditions. It especially helps crypto traders to use speculation to make money.