Loan Against Securities itself states that it is Loan Against Marketable Securities in which customer pledges his investments in favour of lender and borrow funds to meet his financial and personal requirement without selling his investment.
The purpose is to take care of all investment as well as personal needs and meet contingencies. It is the ideal way to get liquidity without liquidating your investments. It would make sense to avail of this facility when you are expecting a certain sum of money a few months down the line and you need some funds in the interim