We have bits of your answer scattered here and there at Ochef.com. But we'll sum it up, because we know your parents don't want you surfing the Internet for hour after hour. Solid vegetable shortening has been around only for about a century, since the process of hydrogenation was developed in 1905. This process bubbles a bit of hydrogen through vegetable oil, which changes its chemical structure, and allows it to be a solid at room temperature.

Margarine has been around only a little longer - essentially since 1870, when it was based on the animal fat suet. Like vegetable shortening, oil-based margarines were only possible after 1905. So the use of butter in cookies - or a solid animal fat - preceded either margarine or vegetable shortening.

One reason you might use shortening in a cookie recipe is that it melts at a higher temperature, so the dough holds its shape longer in the oven, allowing the flour and eggs to set before the cookie collapses and spreads. So it is possible to produce a cookie that does not spread very much. Using butter or margarine (which has a melting point only a degree or two above butter) produces a cookie that spreads out more.

The primary reason you make cookies at home instead of picking one of the hundreds of packages off the shelf at the grocery store is that you can make a cookie that tastes a lot better than store-bought. From the perspective of taste - in our opinion - butter wins hands down. Vegetable shortening adds nothing to the flavor of a cookie, but virtually all store-bought cookies are made with it. Some people prefer the taste of margarine, however, and this is a democracy, after all.