Stable coins that are created on the Ethereum network still rely on Ether to pay for transactions and gas. This is inconvenient for mainstream usage as it means that for a user to pay in a Dai, a stablecoin, they need to also pay Ether to cover the gas. The price of Eth is volatile and the network can be congested which makes the gas prices different from day to day.
With xDai Chain, the user only needs to deal with Dai. With block times of 5 seconds, XDAI can be transferred between different users almost instantly and practically free.