トマト----loanwords/tomato/ food package
マカロニ----loanwords/macaroni/ food package
口夕口夕——Onomatopoeia/ sound of dog’s bark/ a cartoon book
ウチ---emphasis/home/ a cartoon book
In Japanese language, there are three major reasons to use katakana: Loanwords, Onomatopoeia and Emphasis.
Loanwords, which are borrowed from other languages (except Chinese), are mostly written in the form of katakana. Transliteration is way how those borrowed words were introduced into Japanese. For instance, トマト (tomato) and マカロニ(macaroni). These borrowed words are used to define imported things or ideas which did not exist in Japan or its culture previously. Likewise, katakana is also used for foreign names.
There are three advantages to use katakana to translate loanwords: 1) There is no need to create a new Japanese word for these new concepts from foreign courtiers; 2) It is easy for people to distinguish which words are from other languages. 3) Using katakana can compose new syllables that do not exist in Japanese pronunciation system. For example, ティ(ti)、ファ(fa)、ウェ(wai), etc.
Another reason of using katakana is to represent onomatopoeias, a usage which is used to mimic natural and artificial sounds. For instance, 口夕口夕is used for the sound of dogs’ bark. It is easy for people to know that these words are not real words in vocabulary, but only sounds, so it won’t cause any misunderstanding.
In addition, some words will be written in the form of katakana for emphasis purpose, despite the fact that they have equivalents written in Hiragana. For example, the word ウチhas its equivalent term in hiragana, but in a cartoon book the writer uses its katakana form in order to highlight the word and to catch readers’ eyes. I think the special effect of emphasize may come from the angular katakana script.
