It’s programmable, meaning you can use it for smart contracts. A common smart contract use case is dex smart contracts that let you swap between other cryptos and usd without any counterparty risk.
USDT and USDC would be the two biggest examples by market cap, though USDT has been mired in... "controversies", and USDC almost got hit by the US Banking Panic this year,
USDC definitely got hit as it depegged from the USDC for few days and traded for $0.87 at some point, it didn't "explode" (and it restored its peg) only because the US government promised to bail out customers of the Silicon Valley Bank fully and not just upto $250,000 as it would be the case if in an usual FDIC coverage situation. Circle, the company behind this stablecoin had about $3.3 billions in SVB.
That goes for all currencies though. After the Brexit and the possible Grexit, a possible Euro collapse had me worried.
Note that I'm not defending Tether. I have no use for it, I'm just pointing out a bigger picture. The solution is to spread savings across different asset types.
Really "stable coins" are what I would call an attractive nuisance for theft as they call for money sitting around doing nothing "for later" makes a very tempting target. The cryptocurrency space is absolutely loaded with such things. See the number of "banks" and exchanges which suffered from a case of "take the money and run".
Are you thinking of algorithmic stablecoins?