If you don't immediately believe that editing can create tension in a scene, you can watch a re-creation of how the scene was scripted here, but I recommend railing some coke or something first to keep your attention span up. If you can't watch that because you're at work, the pre-Marcia version has Luke taking two full runs at the exhaust port, no appearance of ghost-Obi-Wan telling Luke to use the force, and -- worst of all -- Han scares Vader away from Luke like 45 freaking minutes before Luke takes his shot at the exhaust port, meaning that the only thing we're worried about happening in that scene is Luke missing and having to take the same shot for a goddamn third time. A pretty good rule for filmmaking is that if the biggest threat in your movie is the runtime, then the drama isn't really working.
Basically, without Marcia the movie never would've caught on as anything but another forgettable '70s schlock adventure with a better-than-normal soundtrack, but she was completely written out of Star Wars history by a messy divorce shortly after Empire Strikes Back was released (in fact, the production of Empire had been, in part, an attempt to save their marriage).
In a completely unrelated coincidence, no one made a good Star Wars movie ever again.