OK, here's the story:
The Red Pass key is used on the Assault on Theed level (where you play as the Queen), not the Escape from Theed (Obi-Wan escorts the Queen to the hangar). The artwork is there on both levels, because we used the same level geometry on both.
The reason for the key is if you allowed the Droid to get destroyed (either on the Escape OR Assault levels), you'll need to backtrack. If you talked to the droid on the Escape level, you were given an option to order the droid to follow, and eventually you could send the droid to scout ahead. This almost invariably caused the droid's destruction, and we just wanted to make you go "Doh!" when you realized (when coming back on the Assault level) that the droid actually served a purpose to help you, and his destruction made your path more difficult. It was just a little thing that gave us all a laugh.
One of our playtesters reported that he almost died laughing when he realized that the door couldn't be opened on the Assault level because he caused the droid to get blasted way back on the Escape level.