Initially, the SUI is being staked to earn new issuance (“inflationary”) subsidies. This means that the SUI supply will increase by approximately 1,111,111 SUI tokens per epoch (24 hours) and stakers will capture the newly issued SUI through rewards. It is estimated that stakers will earn around 6% annually. However, the subsidies will decrease every 90 days by 10%. So it would reduce from 6% to 5.40%, then 4.86%, reducing continuously each 90 days. Although the inflationary subsidies will reduce over time, the total Staking rewards rate also consists of rewards derived from network activity / gas fees. Due to low network activity upon launch, Stake subsidies will help make up for rewards until the activity picks up and eventually the rewards will largely be derived from gas fees rather than subsidies.
Stakers will also capture fees from network transactions, so as Sui transaction volume increases, SUI stakers will earn more than just new issuance subsidies.
The SUI token also gives stakers the right to vote on policy decisions for how the Sui network will operate and distribute treasury funds. Owning staked SUI is ownership of the Sui network, entitling SUI stakers to set/change the rules of the Sui network.