Fanfic writers like "mythos building" fics, partly because they tend to be more memorable, but also because they allow them to cement their personal theories into the source material's pseudo-canon; if the fans like the idea, they can collectively adopt it as Fanon.Fans may also try a Fix Fic, where the author identifies something "wrong" with the source material and tries to make it better.As a result, you will need a fraction of the wood to cook up a meal in a stove compared to an open fire.
That said, fan wank can still be enjoyable, especially if the author is clearly aware of how ridiculous the theory is, and some fan theories can even be adopted by the original work in the form of Ascended Fanon (or a Shrug of God, which means no canon commitment either way and gives the fans room to work with).
Years later, a group of Space Pirates called the Gokaigers hear about the Greatest Treasure In The Universe, which is supposedly located on Earth.
Arriving to find and plunder the Greatest Treasure, they accidentally stumble into the Zangyack Empire's second attempt at invading Earth.
This is YMMV for a reason however, as the sheer amount of Fan Wank in existence shows that a lot of people, at least within fandoms, really like it.
See also Epileptic Trees (the specific crazy theories the fans come up with) and Wild Mass Guessing, TV Tropes' own collection of crazy fan theories.