When you sell crypto and want to exchange it for "real" money, say USD, some exchanges don't support that, they only support crypto to crypto conversions. On exchanges that do support it, it's sometimes slow and often comes with a fee. Both the exchange and your bank may impose arbitrary limits on it. It may create a taxable event.
By comparison, a crypto to crypto conversion (say BTC to USDC) is instant, often free, and without much limitation.
If actual USD had none of the above limitations, nobody would need or use a stablecoin. It's a utility, not some bet against the dollar.