I think it's used to talk about a future event that's not going to happen. I don't know how to explain it grammatically, but I can give you an example. Imagine you're having a birthday party and you're inviting your friend. If your friend hasn't decided if he's going or not and you're trying to persuade him to go, you can say, "You have to come; it will be so much fun." If your friend has already decided not to go, you can say to him, "It's too bad you can't make it; it would have been so much fun." I'm not sure how to explain it, but in my mind, even though the party hasn't happened yet, since you already know your friend won't be there, you're saying it from the perspective after the party and how it would have been fun if your friend had been there. I'm not sure if that makes sense to you, but it's how I perceive it in my mind. Another example is: "If I hadn't quit my job, I would have been working for that company for 10 years next month."