"What day did the Lord create Spinal Tap, and couldn't he have rested on that day, too?"
Nice burn, Marty.
The character of Marty DiBergi is one-of-a-kind. An actor (Rob Reiner) turned director is the director of this particular film, but portrays a second, fictional director within the film. Head swimming yet?
Welcome to the rabbit hole that is the world of the mockumentary. Rob Reiner wanted to be directly involved in the film, more than by just pointing a camera at his funny friends and yelling "cut" every so often. After all, he had the credentials to back it up. Reiner was one of the stars of All in the Family, one of the most popular sitcoms of all time. So he wasn't just some director who vainly wanted to pop himself into his own film because he could. Looking at you, Mr. Shyamalan.
And the inclusion of the director/interviewer within the film wasn't just a throw-away, either. Many other documentaries of the time were introduced by the film's director, who appeared on camera, and maybe even included footage of the director interviewing the film's subjects. So this technique lent an air of authenticity and made Spinal Tap seem more like an actual documentary.
MARTY: I wanted to capture the... the sights, the sounds, the smells of a hard-working rock band, on the road. And I got that; I got more, a lot more. But hey, enough of my yakkin'; whaddaya say? Let's boogie!
Through his one-on-one interviews with the band members, Marty's the vehicle through which the audience gets a ton of information about the inner workings of the band members' minds—at least what's left of them. He doesn't shy away from the tough stuff, getting the guys' reactions to bad reviews and probing into the very bad luck of a series of Spinal Tap's drummers.
We get to see the band's rise and fall (mostly fall) through the eyes of a third party who seems to think they're all as ridiculous as we do.