A useful currency should be a medium of exchange, a unit of account, and a store of value. Cryptocurrencies excel at the first, but as a store of value or unit of account, they’re pretty bad. You cannot be an effective store of value if your price fluctuates by 20% on a normal day.
This is where stablecoins come in. Stablecoins are price-stable cryptocurrencies, meaning the market price of a stablecoin is pegged to another stable asset, like the US dollar.
Source: Stablecoins: designing a price-stable cryptocurrency, Haseeb Qureshi, February 19, 2018