Stablecoins rarely ever fluctuate in prices, flashbacks to when the usdt halved in price are irrelevant but worthy of noting here due to the fact that it had experienced a hack - something that every cryptocurrency is ultimately vulnerable to.
Regardless, USDT is relatively stable not to mention the fact that there are easily billions of dollars being moved in and out of usdt every single day and even more that are directly transferred to bitcoin and ethereum in trading pairs and so on.
These stablecoins react to genuinely destabilizing events in the crypto world - for example, if the usdt ever gets hacked into; we'd obviously get f*****d. Or if the world trade towers were attacked again, we'd likely see the same 2008 situation reoccur just because that's how human psychology works. There'd be everyone losing money and some gaining money like the war-based companies and gun companies that have open contracts with the united states military.
Stablecoins are used for digital loans since they closely correlate to the us dollar and are commonly traded within finance spaces. They're also commonly used to tackle regular payments on digital purchases and general selling of goods.