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.