Shimmer is a test network for IOTA but it acts like a real network and has own token SMR and rewards the developers and users. All the functionalities that work well on Shimmer will be implemented on IOTA.
Assembly is another family member of IOTA, I understand Assembly features will be tested on Shimmer network as well. Assembly is validator marketplace for smartcontract chains to be decentralized.
In the future (after Shimmer network is launched which is upcoming on 28-Sep), we can stake Shimmer(SMR) for SMR rewards. In the mean time we can continue staking IOTA for Assembly tokens and after Assembly launched, both IOTA and ASMB can be staked for ASMB rewards.