- Most of the time it's simply a pragmatic way to introduce a clarification without breaking the flow of the text, essentially a more concise form of parenthetical or footnote.
- In classical poetry it is used for a variety of effects, for example novel synecdoches. One side of the gikun might refer to a season, and the other side might refer to a key detail the poet idiosyncratically associates with that season.
- But the contemporary Japanese learner usually notices them the most in fantasy/sci-fi manga and novels. In this genre it's used to introduce in-universe jargon while showing its meaning in parallel. At the extreme, it can allow writers to go over-the-top with how much special jargon the universe includes, without slowing down the pace of storytelling. (This can pose quite a challenge for translators!)
In the original Chinese language, a "word" mostly consists of a single character. Interestingly, many of the compound words commonly seen in modern Chinese were in fact coined by the Japanese scholars during their attempts to translate western writings around the 19th century and were later "imported" back into Chinese language. Interestingly, the two examples in the article, "art" (美术) and "science" (科学) are both of Japanese origin, though one can still tell whoever coined the terms chose the individual characters due to their meaning being relevant to the concepts the words are describing.