To under stand the function shortening has in a recipe you need to understand what shortening is first. Shortening is edible fat (in nature) at room temperature. Since shortening is 100% fat it lacks water, so it doesn’t increase gluten production. Meaning cookies made with shortening tend to be softer and more tender. Also, shortening has a higher melting point, so the flour and eggs in the cookies have a longer time to set before the shortening melts, resulting in cookies that are taller and not as flat. This is important to know because no one wants flat cookies, or at least I know I don’t.
Source: Baker Pedia: Shortening