The day traders, the speculators need them.
Since cryptocurrencies' price fluctuates (against dollar) they exchange their crypto in and out of these stablecoins and gain some % here and there (and they don't touch fiat money, so less tax issues, more exchanges they can work with and so on).
Also they can easily do arbitrage (buy at one exchange and sell at another) since transferring stable coins works much faster that with fiat money.