Katakana Analysis

Loanword, インフルエンザワクチン(Flu vaccine)
Loanword, マニフェスト(Manifest)
Loanword, ハイライト(Highlights)
loanword, ラーメン (a kind of noodle)
sound, ピンポン(the door bell)
sound, ビュービュー（whistling sound）
Technical terms, プラチナ(platinum)

The katakana words are most commonly used for transcriptions from foreign language, as far as I concern, especially transcriptions from English words, such as インフルエンザワクチン(Flu vaccine). And sometimes it is also used to write Chinese loanwords, such as ラーメン (a kind of noodle). To use katakana in this way can keep the words pronounced in their original way, and make difficult-to-read kanji words easy to read.

The katakana words are also used to describe an onomatopoeia or sound, such as ピンポン(the door bell) and ビュービュー（whistling sound).

Katakana words are also used in technical terms, such as in metallurgy, プラチナ(platinum). Maybe people use these words in Katakana to distinguish the specific usage from the words used in daily life.

People also used katakana words to emphasize specific meaning or call for other's attention in Katakana. For example, write an important anouncement in Katakana, just as write in English with all capital letters.

Different textbooks explain the usage of Katakana in different way.

First, because Katakana can be used in a lot of ways due to their multi-functions.

Second, maybe because the development of Katakana words is really complicated so people can understand it from different perspectives.