Generally MacGuffins are explained, even if they have no inherent value in of themselves, only by what people are willing to do to get their hands on them. Like the letters of transit in Casablanca or the Maltese Falcon in...well, you know.

My guess is the MacGuffin in Pulp Fiction, by not being explained, is reminding the audience that it is only a MacGuffin, and the contents don't really matter. It's a sly wink to the audience, as it were.