As stated in the tweet, the move was made to benefit users. Through this merger, users can deposit stETH as insurance in Aave to earn interest. stETH is a token depicting staked Ether in Lido. These tokens are minted upon deposits and burned while redeeming.
Users can also add stKSM to the largest DeX on Moonriver's Kusama-based para chain, Solarbeam. Through the Solarbeam pool, users can earn rewards. DeFi Saver tweeted, on Monday, that there are only a few stETH deposited to date and there haven't been any borrowings as of now.
On a secondary note, Lido took to Twitter to share that they have staked 2 Million ETH with Lido till now.