Home Movies in Film

Home movies like to appear in fictional films often. There are of course, exceptions, the best of which being a documentary that utilizes home movies, called Dear Zachary. However, what is their function in a fictional film, where they are obviously contrivances that are supposed to show that "these things used to happen" or "these people are like yourself". And that is perhaps the central problems of this utilization, for it assumes that we all watch home movies, when only some of us do, and that's probably half-heartedly.