Liquidation bonuses exist in order to incentivize liquidators to help keep Solend solvent, providing a buffer to absorb loss from slippage. Liquidators have been well-compensated for this service, racking up an estimated $3M in profit in the first year of Solend being live on mainnet. Solend takes on the risk that liquidations are unprofitable, creating bad debt. In the event of bad debt, Solend's insurance fund is tapped to make users whole. Close to $1M has been paid out from Solend's insurance fund for such issues in the past. Taking a fee allows Solend to build up its treasury so it can continue providing users with insurance.