Displaying merchandise in a storefront, especially when your store is in a high traffic area is a great way to market. You don't necessarily have to keep all your merchandise on display, just one or some of each, or the newest merchandise. This will catch the eye of people walking or driving by, and if it something they are interested in, or in the market to buy, they will most likely walk into the store, and possibly purchase something.
Whereas if you keep all your merchandise in storage, it doesn't have any marketing value at all. Potential customers aren't seeing it, and it will be an almost unknown product, causing you to lose money on it.