Wikipedia:Germane

In order to produce a rationally divided encyclopedia, each article bears a title. The idea is that material found under that title has a substantial relationship to that title – else the title becomes ineffective for the use of users of Wikipedia.

Articles filled with "interesting facts" which are not reasonably related to the subject of the article are not actually very useful at all. One example cited would be inclusion of a section about Santa Claus in an article about Whales unless a truly creative connection has been made by reliable sources, and then only to the extent the reliable sources connect the topics.

Trickier still
Sometimes an editor adds content which is related to the topic. However, it does not relate specifically to the topic. For example, in Art film, at one point an editor added a summary of the key points of film theory. Film theory is related to art film, as film scholars use film theory to describe what makes a movie qualify as an "art film". However, generalities about film theory are not needed in the art film article. If there is going to be film theory content added, it should be "film theory about art film". See WP:PROPORTION for more information.