I've had a soft spot for stories told as found documents since I first watched the Blair Witch Project in high school. And, yes, while there are legitimate criticisms of movies made in that kind of shaky-hand, “no this is real, though” style, I do appreciate the extra level of immersion it brings. It's a full commitment to the lie. Typical movies say, “Here, watch this idea I came up with and let's pretend together that it's something that happened.” But when it's found footage, the person behind the camera is saying: “This really happened.” And no, they don't really believe that—but there's more of a sense that they want the reader to believe it, more of an invitation to inhabit the story's world.
The same applies when the story is told in book form. High school was also around when I first read other people's published journals, Anne Frank's first for class then Sylvia Plath's on my own. Real-life journals are even more intimate than a memoir or autobiography. The author isn't curating their life to present it to the audience. They're showing their inner thoughts, day-to-day, exposing their full, theoretically uncensored selves on the page.