"For one thing, you're clearly a dog."
This attitude is particularly common when it comes to creative stuff. A friend dismisses your life-size mural of a kitten riding a dragon; they could do it better, you know, if they wanted to. Relatives claim that playing the harmonica must be easy because you do it so well, ignoring all your long nights slaving away at harmonica school. Movie making is no different: A lot of people have the attitude that a director just points cameras at stuff, and everything else works itself out.
This is where bad movies come in. Most good-bad movies were made by the film industry version of the waffle-eating guy picking on your Frisbee game. For example, rumor has it that the famed 1966 good-bad movie Manos: The Hands of Fate was actually the result of a bet somebody made with the director after he boasted that movie making was no big deal, anyone could do it. The result was this:
Watching Manos is like attending a reverse filmmaking school. It's a level of bad that most of us non-filmmakers never imagined could exist, because we've never seen that far behind the scenes of that particular creative process. Moths fly at the screen because of the lights the crew was using at night. Entire scenes are out of focus. Whole shots are blocked by the back of an actor's head. Manos is an extreme example, but most good-bad movies tend to have been vomited out by people who both overestimated their own skills and underestimated the difficulty of making a good film. Watching these movies is like vicariously dragging Waffle Taco Guy up and forcing him to throw a Frisbee at a professional level, and then watching him cry as his Frisbee goes off course and knocks over a fruit stand.
Furthermore, good-bad movies teach us to appreciate qualities in movies that were formerly invisible. Sure, they have a whole Oscar category for sound mixing, but if you really want to appreciate what good sound mixing does for a movie, watch this scene in Birdemic fail at it. We might laugh at a wooden performance or two in a blockbuster, but we don't realize just how good most actors in movies are until we watch The Room. Sure, the last Hobbit movie kind of sucked, but at least no one got hit in the face with a boom mic.
C. Coville has a Twitter here and a Tumblr here.