The Uniswap pool is a good, decentralized way to provide on/off ramps to the synth system. Changes in the exchange rate are bad for Uniswap liquidity providers (impairment loss). Since the sETH/ETH ratio should be pretty much constant, that pool is ideal for liquidity providers: there is no impairment loss so the liquidity providers should end up getting all of the exchange fees. To make it even more enticing for LPs, some of the SNX inflationary reward has been diverted to them. The result is a super deep uniswap pool, which is good for the synth ecosystem. Before the Uniswap pool, the main way to get in and out of the system (other than minting) was to use thin markets on centralized exchanges.