Katakana Analysis Draft

The following are several examples I find.

1. Though loan words from ancient Chinese are usually written in kanji. For loanwords from modern Chinese, katakana is used.

Japanese	Rōmaji	Meaning	Kanji	Romanization	Source language
マージャン	mājan	mahjong	麻將	májiàng	Mandarin
ウーロン茶	ūroncha	Oolong tea	烏龍茶	wūlóngchá
チャーハン	chāhan	fried rice	炒飯	chǎofàn
チャーシュー	chāshū	barbecued pork	叉焼	cha siu	Cantonese
シューマイ	shūmai	a form of dim sum	焼売	siu maai

I think the reason why this is the case is that the modern Chinese words can not be well or correctly understood  by Japanese people. The kanji Japanese people are using almost keeping their original meaning in ancient Chinese, while we Chinese add a lot of new meanings to Chinese characters. Even though some words are written in the same way, we have different understanding with Japanese. A simple example, "勉强" as kanji in Japanese it means study, however in Chinese it means “force somebody to do something he'/she does not want to do”; totally different meanings! Given there is no advantage of understanding for use kanji in these loanwords, I think katakana is more convenient for reading.^^ 

2. Katakana are also used for emphasis, especially on signs, advertisements, and hoardings. For example, it is common to see ココ ("here"), ゴミ ("trash"), or メガネ ("glasses").

Words the writer wishes to emphasize in a sentence are also sometimes written in katakana, mirroring the European usage of itlics.

3. ヒフ科("dermatology"). 

Some difficult-to-read kanji are written with katakana. For this example, the kanji is "皮膚科". The second kanji is rather complicated and difficult to read. Thus this word is commonly written as 皮フ科 or ヒフ科, mixing kanji and katakana.