Traditionally, margin loans were used by investors to allow them to buy additional shares of stock. Recently, however, many investors have used such loans as a simple way to borrow money for purchases outside their investment portfolio. Even so, they are still subject to margin calls should the value of the securities pledged as collateral fall. See article here, WSJ.
Dolvin, Steven D., "Margin Loans" (2015). All Chapters. 141.