Stablecoins are useful, because they let you set prices on the blockchain that aren't denominated in BTC or ETH or another currency that fluctuates wildly. A good stablecoin is pretty boring in terms of gains/profits, but can generate a modest stream of fees.
Terra had a sister coin called Luna which it planned to use to keep Terra stable. Luna had the price dynamics of ETH or BTC, where it's value depended solely on the whims of the market. The whole thing was a perpetual motion machine that failed very predictably. Matt Levine has some good coverage.