If one is using something like Uniswap to exchange crypto, you cannot go directly to Fiat, so people that want to sell a token or Ethereum will trade the asset

If one is using something like Uniswap to exchange crypto, you cannot go directly to Fiat, so people that want to sell a token or Ethereum will trade the asset for USDT and then later trade the USDT for another asset when the price has dropped. It's just a way to buy and sell crypto with an intermediate asset that has almost no price volatility.