Staked STRD receives 85% of protocol fees, which is distributed as a basket of different Cosmos tokens. So stakers receive stTIA, stDYDX, stATOM, etc as a staking reward. While the majority of the staking reward is from protocol fees, a supplement of newly-emitted STRD is added.
Governance
Staked STRD can vote in onchain governance proposals. Stakers can vote on: which host-chain validators to delegate tokens to, how to spend community pool funds, protocol upgrades, and other important issues.
Securing the network
By staking their STRD, users are effectively securing the integrity of the network. For that, they earn some additional APR through inflation.