I've thought about making a post like this a lot, because I read a lot of badfic, and I talk to a lot of people who don't, and it's something that most people don't understand. The question comes up directly, or by implication, a lot.
Reading badfic - even though I often choose good writing over it when I have the chance - is something that I enjoy for its own sake even when it does pain me, and not just in the "there was no good fic to read" way. It's a different experience, a difference in kind and a difference in degree at once. If reading good fiction is a dream about swimming deep underwater, then reading badfic is a thoughtful trance while you wade through a stream. Perhaps a more apt analogy is that reading badfic is like looking at child art, although strictly speaking the badfic of the unsophisticated adult and the child-like art of the unsophisticated adult differ in some significant ways from the art and writing of actual children (although certainly you find the writing of actual children in among badfic. Even in fandom, as their authors' notes so horrifyingly notify me).
In some recent meta posts like
the one on access character and heroinisation I talked about how the body of fanon found in badfic lies closer to the fannish collective unconscious. In a recent meta post by somebody else associated, I think, with the "does fanfic make us poor" debate (unfortunately, I can't remember which), fanfiction was likened to primitive writing or storytelling. I would argue that this isn't true of all fanfiction, but it is true of badfic. Looking at a fandom's badfic shows you, eventually, a gradually emerging picture of the fannish collective unconscious as it applies to that fandom; it shows you the first storytelling impulses of many fans unfiltered by the additional processes which go into writing more sophisticated fiction.
At a certain early stage, a child's representations of other people are almost wholly symbolic. As they perceive more things about people they add more details: lips, ears, eyelashes, fingers and toes. You get these blob people, circles with eyes and lips and noses but usually no ears or eyebrows, with spindly sticks emerging from the sides (where the ears would be, often) and from the bottom for the arms and legs, with circles for feet and hands and stick fingers. There are almost never five sticks on hands or feet. The child considers it important to represent that the person
has fingers, and a nose, and a head; the number of fingers is obviously less important. In other words, child art tells you about how the child looks at things, but it also tells you about how people look at things, because people used to be children. People learn to think symbolically first. People fasten onto the fact of fingers before they reach the more sophisticated stage of noticing their number.
In the same way, badfic shows you how the badfic writers look at things, which details they fasten onto, but it also shows you a glimpse of the fannish collective unconscious, patterns which emerge in good stories as well.