Although charging the borrower interest isn't necessary, it is a way for the lender to make money on the loan, as well as a way to provide compensation to the lender for the risk involved with lending money to a third party.
LawDepot's Loan Agreement allows you to include compound interest, which is interest calculated based on the original loan amount and the interest that is accumulated from previous periods. You can choose whether the interest is compounded monthly, every six months, or annually.