Developers choose xDai to run their Decentralized Applications (DApps) for several reasons.
Transactions process extremely quickly.
Fees are small and stable (allows for micro-transactions and the ability to accurately predict and plan for fees).
Reliability and security.
Projects can also leverage TokenBridge tech to create DApps that utilize the xDai network for "heavy lifting", then move assets procured on xDai to the Ethereum Mainnet.
A short list of DApps submitted to State of the Dapps (this is not an exhaustive list, only apps submitted to their system) is available at https://www.stateofthedapps.com/platforms/xdai