The initial driver for the securities lending business was to cover settlement failure. If one party fails to deliver stock to you it can mean that you are unable to deliver stock that you have already sold to another party. In order to avoid the costs and penalties that can arise from settlement failure, stock could be borrowed at a fee, and delivered to the second party. When your initial stock finally arrived (or was obtained from another source) lender would receive back the same number of shares in the security they lent.