I assume for the usual reasons argued in favor of cryptocurrency: anonymity and decentralization. But the exchange rate and contraction/expansion algorithms presumably must be consensus. And the more heuristics you constrain the system with at the start, the greater the likelihood of forks down the road.

A "stablecoin" allows traders to move in and out of cryptocurrency positions to a stable currency quickly, without converting to true USD, which is slow to process.

It also allows the transfer of stable value (payments, remittances) on the blockchain, rather than transferring volatile currencies and converting them to USD, incurring the risk of price fluctuation and conversion fees.

A stable coin is a very valuable asset to the ecosystem.