
There are a lot of characters, both in the GFFA and here on Earth, that are basically essential to Star Wars as a popular saga. I can't say there is a single thing that "makes" the movies, because there are just so many things that do. Max Rebo, we can live without. But there are somethings that just make the movies into the marvelous thrill ride that they are.
To start off this series-to-be of blogs highlighting the vitals of Star Wars, I'd like to recognize the coolest mute trashcan to ever grace the silver screen, R2-D2.
R2-D2 is a great character all around. He's talented, he's funny (which is pretty impressive given that he can't talk), and he's a friggin space robot with a built in jetpack. But being cool as a Hoth winter doesn't make one vital to the saga.
R2-D2 and C-3PO together are the linchpin of the saga. They're the observers, the followers, the heroes, the tools, the plot points, and the comedians that remain constant throughout the entire saga. The interactions of those two artificial beings provide much of the life of the saga. "R2 and 3PO" are the hook of ANH, the birth of the Star Wars story. They were there at the first space battle we ever saw, and their little arguments and their weird mission kept us us watching until R2 introduced us to Obi-Wan. It's subtle, but symbolically important, I feel.
R2 himself is also of great practical importance to the characters of the saga. That little guy can't go one movie without saving the skin of some or all of the "main" characters. But it's not just that he saves everyone, it's what he saves him from. Impossible situations, resolved in a moment by one brave little droid.
The suspense of Padme being covered in lava, the tension of the Falcon being unable to jump to lightspeed, the sheer horror of Luke being forced to dangle above a Sarlacc pit! None of those intense dramatic scenes would be possible without some convenient and unlikely, but simple and sudden resolution to change the whole picture in a moment. The drama, the swings of action, made possible by R2-D2, and he carries it all out so elegantly, you wouldn't even notice that all hes' doing is magically resolving impossible situations by poking something with a stick.
R2-D2 is a rolling Deus ex Machina.
That's R2's real importance. There's only one other thing in Star Wars with comparable ability to make such drama possible. And that's the Force. That's right, R2-D2 is as powerful a plot device as the Force itself. You heard it here first.