What psychological function do conspiracy theories serve?

There are a lot of people who believe in various conspiracy theories, and many of them are upstanding members of their communities. People who don’t believe those theories might laugh and scoff and call the believers kooks, but people wouldn’t believe in them if it didn’t serve them in some way.

Why do people believe in conspiracy theories? What do they get out of it? What can you say about the meaning or function of these theories in believers lives?

If possible, reference to specific theories or categories of conspiracy theories would be nice.