Conspiracy theories are mostly junk.

But there are two classes that sometimes hold water.

First there are the organizational cover-ups. Something bad happened: "let's sweep it under the rug or set up a committee to report back in ten years' time" is a common response. Which sometimes turns evil ("let's silence the witnesses").

And then there are shared interests. Own a coal mine? Worried about environmentalism? Donate to climate change skeptics! (Repeat, for all the fossil fuel industries.)