The goal is to have zero correlation to the index (called beta). You’re hoping that your long assets will go up in value regardless of what the index is doing (the alpha of the stock). Similarly with your short positions, you’re hoping that they are going to go down regardless of what the index is doing. In a sense you’re eliminating “chance” by driving the beta close to zero and instead are focusing more on what actually makes the company a good investment (it’s alpha)