High gas fees on Ethereum make many applications more difficult to use. This includes DeFi applications - notably DEXs like Uniswap and others. Often, fees can eclipse actual trading amounts, and can limit a trader's ability to capitalize on opportunities. Trading on xDai allows users to make many swaps, or create many positions, without fee prices limited the number of trades. Faster blocks also enable more rapid trading options.
There are costs associated with moving assets to xDai (and bridging them back to Ethereum). But once they have been moved, trades can be executed as often as a trader desires.
HoneySwap, a Uniswap fork on xDai, enables fast and inexpensive trading.
Perpetual Protocol allows traders to go long or short on many different types of assets. 0 fee trading for users, as xDai fees are small enough to be subsidized by the protocol.