Press "Select" while selecting a mission item to see what it does. Their effects are usually small, but they are there. And some have very powerful effects, such as the Insignia, which doubles the missions AP, and the Ally Finder 2, which makes new additions to your clan belong to more advanced jobs (and while they won't have any abilities mastered, it is still very useful, especially as you get to higher levels. A level 50 Fencer will have very different stats from a level 50 Assassin).

Oh, and I forgot to mention something earlier. There is another way to get the Excalibur 2. I won't say what it is just yet (since it is basically the same way you get the Nagrarok outside of the mission, and that is my current trivia question), but it is possible.