The way to protect yourself from volatility with the help of stablecoins is that you can sell your BTC for USDT if you think its price will fall - and that is something that a lot of people practice and I have often read such comments on the forum. Of course, a similar thing is achieved if you sell crypto for fiat, but here we return to the fact that people want something that allows them to dispose of their funds very efficiently and quickly - and this is exactly the reason why stablecoins have become very popular.
Secondly, the explosion of coins on multiple chains means that their trading pairs are available only with these centralized stablecoins (which are accessible cross-chain due to the central nature). I am not sure if this has been a good thing though. Earlier, most trading pairs on major exchanges were pegged only to BTC or ETH. This meant that most newbies came in through BTC (acquired in a privacy centric manner), and the shitcoin jugglery was done chiefly to increase your BTC holdings.
Now with different chains and the USD pegged stablecoins, all roads now seem to be leading to Rome, at least when it comes to the Alt-coins on other chains like Polkadot, Solana etc.