These fees compensate the validator network for the CPU/GPU and network resources necessary to process the state transaction. Transaction fees are extremely cheap (currently 0.000005 SOL).
These fees compensate the validator network for the memory resources that are required to store state in an account. Accounts which maintain a minimum balance equivalent to 2 years of rent payments are exempt from paying rent. When solend.fi is used, accounts are allocated on behalf of the user and funded with enough SOL to be rent-exempt. This amount can be recuperated later when the account is closed (though this isn't yet implemented on solend.fi). Rent fees can be expensive compared to transaction fees (~0.01 SOL), but are only paid the first time a user is interacting with Solend.