Right now the only way to earn interest on your Bitcoin is to use a CeFi (centralized finance) crypto bank like BlockFi. Once you "wrap" bitcoin and turn it into an ETH token, you can use that value in DeFi. And since Bitcoin has the largest market cap of any digital asset, this would pump the DeFi crowd's favorite vanity metric "amount locked in DeFi".
It also doesn't hurt that Bitcoin has slower blocktimes and higher fees, so technically you could transact Bitcoin "easier" and "cheaper" on Ethereum. Although ETH will run into those same speed/cost issues eventually to.