Stablecoins such as USDT and USDC are the cash of crypto markets: most trading pairs are quoted against them, and moving to a stablecoin is how traders go to cash without leaving the exchange.
Their risk is the peg. Reserve-backed coins depend on the issuer holding real assets; algorithmic coins have collapsed to zero (TerraUSD in 2022).
Example: selling 1 BTC for 60,000 USDT lets you hold dollar-like value on a dex or cex and buy back later, but you are now exposed to the stablecoin issuer rather than to Bitcoin.