The principal reason for borrowing a security is to cover a short position. As you are obliged to deliver the security, you will have to borrow it. At the end of the agreement you will have to return an "equivalent" security to the lender. Equivalent in this context means fungible, i.e. the securities have to be completely interchangeable. Compare this with lending a ten euro note. You do not expect exactly the same note back, as any ten euro note will do.