Stable coins usually have a constant value compared to fiat, and that creates their value in the market.
Stable coins are the simplest way for a new investor to enter the crypto market: they use fiat to buy stable coins and find opportunities. So we can observe stable coin's Cap to determine investor interest in market. Currently, the stable coin's Cap has increased from $121B to $128B, suggesting the possibility that crypto market has reversed from a decrease to an increase.
Stable coins are also a safe haven for investors as soon as they exit crypto without wanting to leave market. Without stable coins, entering the market would be extremely risky.
Stable coins make market easier to understand and have better liquidity: compared to the period before 2017, traders can only participate in ALT/BTC and BTC/USD pairs, which is really inconvenient when they want to calculate profits in fiat. Currently, the Vol of ALT/stable coin pairs is dominating, showing that stable coins continue to be chosen by investors.
Stable coins are also involved in lending services, margin, futures... I really can't predict how bleak these markets would be without stable coins.