Law of Wiki Expansion

Any specialised wiki, as it continues to grow, will tend to accumulate general, more encyclopedic, information.

This dates back to the original wiki, Ward Cunningham's original Wiki Wiki Web. There were enough Separated by a Common Language problems that the users felt the need to create a page explaining "American Cultural Assumptions"... which turned out to be almost exactly like our Useful Notes pages.

For every wiki out there that exists by The Wiki Rule there is the issue of some additional knowledge not exactly under the main mission of the site still being very useful for understanding the behaviour of the site's focus. There's also the fact that the internet exists globally while a wiki's contributors can exhibit local variations in behaviour and these often have to be explained.

For example, our Useful Notes pages exist to help people understand things that may be taken for granted under the Creator Provincialism of other contributors or of TV writers. It started with issues of geography and then extended to subjects that have a complex relationship with their portrayal in media. Things which it is useful to know in order to understand everything that is written hence Useful Notes. Even before this, we also have pages on things which are not in fact tropes but which still impact on how stories are written such as Executive Meddling and the Fandom tropes.

Examples of Law of Wiki Expansion include:

Halopedia has a page on corn, as well as other things, due to it being an inclusive wiki.
The Transformers wiki has a lot of information on toy production and technology. The Transformers Wiki also has pages for Jesus (probably existing to service the comparisons between Optimus Prime and Him) and Michael Jackson.
The Star Wars wiki includes pages on gender and death, among other things.
TV Tropes used to stick to television, hence the name. Now it covers tropes from all conceivable media, and some things that aren't technically tropes at all.
Since Donkey Kong exists in the same universe as Super Mario Bros., it isn't surprising that the Super Mario Wiki would have some information on the DK characters and games. However, it has extensive information on everything Donkey Kong-related, even more than the actual Donkey Kong wiki. The wiki even has pages for characters in series such as The Legend of Zelda, Metroid, Fire Emblem, and many others due to their inclusion in Super Smash Bros., which the wiki kinda treats as a sort of Mario subseries/spinoff.