Even if you know where things are going to end up, the journey can still be a lot of fun. If the storyteller is really good then "knowing" the end can even heighten the tension. In Romeo and Juliet you know everything is going to go to hell, but finding out how is exciting. In a war movie you might "know" that all the protagonists are going to die, and the tension becomes whether they'll be able to complete their goal before they die, or even just examining how they conduct themselves in the face of certain death. In a romcom where you know the characters are going to end up together you can get to a point in the third act where it seems like they're going to be separated and the tension becomes finding out how they resolve the conflict and get to live happily ever after.