In One Piece for example I've noticed that the word "Log Pose" is written as 記録指針｛ログポーズ｝. Another example, also found in One Piece, is the word "Poneglyph", written as 歴史｛ポーネ｝の本文{グリフ｝.
Why do words such as these have their own kanji readings despite being loanwords or katakana words?
I believe I've seen a similar case in JoJo's Bizarre Adventure manga but I cannot recall which word.
One Piece is set in a fictional world, but the setting is clearly not Japan. Actually, apart from Zoro's swords I can't remember anything that is related to Japan.
In this world 記録指針{きろくししん} sounds like a poor translation of some "original" word; ログポーズ sounds much more authentic, but you have no idea what it means (not even if you figure out the English should be Log Pose). Explaining each term and asking the author to remember its meaning is not impossible, but it would take a lot away from the development of the actual story. (And what would happen if someone starts reading at volume 10 and misses the explanation?)
Writing 記録指針{ログポーズ} allows for the author to use loanwords, like ログポーズ with the meaning 記録指針 right next to it, all the time, everywhere.
Some writers like to do it because it adds meaning to what may otherwise be incoherent sounds. This works even for me, a native English speaker: I've never read any One Piece, so I have no idea what "Log Pose" or "Poneglyph" mean. But if their names are written as kanji with furigana applied, I can take a guess at what kind of thing they are. It works even better for a native Japanese speaker who might not know any given English word (though in this case you wouldn't understand what the names meant anyway). A lot of writers will do this, and it's not limited to manga; I've seen it happen in multiple video games.
