What's the actual purpose of DAI?

AFAIU a stablecoin like DAI requires another crypto as collateral. If the value of the collateral fell, the borrower would either have to top up the collateral or have the vault liquidated. Either way, the borrower would suffer losses caused by the volatility of the collateral. So what's the point of borrowing DAI if it can't shield the borrower from volatility? What does the borrower gain from borrowing DAI? Is it just to allow the borrower gain access to USD without having to sell the collateral? Is all this just for tax efficiency and the opportunity to benefit from any appreciation of the collateral?