The Briefcase is a MacGuffin (sometimes McGuffin or Maguffin)which is a plot device that motivates the characters and advances the story, but has little other relevance to the story. The element that distinguishes a MacGuffin from other types of plot devices is that it is not important what object the MacGuffin specifically is. Anything that serves as a motivation will do. A true MacGuffin is essentially interchangeable. Its importance will generally be accepted completely by the story's characters, with minimal explanation. From the audience's perspective, the MacGuffin is not the point of the story.
The technique is common in films, especially thrillers. Commonly, though not always, the MacGuffin is the central focus of the film in the first act, and then declines in significance as the struggles and motivations of the characters take center stage. Sometimes the MacGuffin is all but forgotten by the end of the film.
Because a MacGuffin is, by definition, ultimately unimportant to the story, its use can test the suspension of disbelief of audiences. Well-done works will compensate for this, with a good story, interesting characters, talented acting/writing, and so on. Inferior films, which fail in those areas, often only highlight a MacGuffin, sometimes to the point of ridiculousness. MacGuffins may be acceptable to the general audience, but fail to be believable for experts in the subject matter (such as a particular technology, or historical detail).
Tarantino mentioned the film repo man starring emilio estevez as the inspiration for this macguffin as a similar gold glow appears from the boot of cars.. also a macguffin..