Many exchanges, especially those that offer any type of contracts trading, are not under normal financial regulation and to the extent they even offer fiat denominated orderbooks there is a risk premium attached to it. When withdrawals are not guaranteed by by any type of regulatory body people could just as well trade pretend-dollars, and that's basically what stablecoins are.
Be cautious out there. At least these holders got their money back.
People who want to invest/speculate in cryptocurrency will often want some proportion of their portfolio value to be as stable as USD. But, actually holding true USD at exchanges/banks could be more complicated, requiring interfacing with legacy banking systems & regulations. A stablecoin allows USD value to be handled at exchanges (and transferred) in manners almost wholly analogous to other cryptocurrencies.
This simplifies, eg, a dApp hiring staff: you hire them for stable coins, which they cash out at various certified brokers.
1. A base "asset" to hold with nearly instant ability to move, ideally without changing value relative to the reporting currency.
2. Moving assets around between exchanges/markets/wallets outside banking hours (9a-5p M-F).
3. Banking one-self (no intermediary ever required), the owner can determine the appropriate security procedures for their assets (granted this may well be not on target of the use-case...)