I found three categories of katakana that I can focus on to start my analysis.
The first kind of katakana word that i find interesting is グリーン . I saw this word on a packaging that is made of paper. I think it is written in katakana instead of kanji or hiragana because the word and concept (of environmentally friendly) is loan. Maybe the translator thinks that everybody knows or should know what green means so it is not necessary to translate it to the Japanese words for "environmentally friendly" or "midori"(in which case it makes a wrong emphasis). By using katakana, the usage of the word "green" can therefore be "promoted" to people who do not know what "green" means. A second reason could be that for the purpose of saving cost in production, packagings use katakana instead of other words. Using english directly would be not be appropriate here, because for industrial standards, they may need to use official language. Similarly, another word in this category is ウィキペディア . It is a well-known word in other parts of the world, so to use a translated name does not help with the promotion of this website. In general i think, often when katakana are used for loan words, such as for brand names, technology words and news topics, they are used for continuing the spread of a certain concept that is popular elsewhere, so that those who don't know yet can keep in sync with the world. More examples are マクドナルド, or スマートフォン.
The second type of word I am going to discuss is ラーメン, like many words for food items, this word is a loan word, but it has a kanji version as well. Often we see it written in katakana instead of kanji. I think there maybe several reasons behind it. First it is faster to write katakana, and easier to remember. Besides that, perhaps it is also more suitable to be written in katakana than kanji, because of the affordable price of ramen itself. Maybe some ramen shops would like to use kanji to gain attention. But I guess initially people used katakana for the word ramen because ramen was a "simple" food, so it didn't look like it would suit a complicated kanji name (again for promotional/marketing purposes I guess).
The third category of katakana is of words such as,カワイイ, that can be found in manga or television programs, and were originally in hiragana form. Let's take カワイイ as an example. In this case, I think katakana is used to emphasize the cuteness, or to emphasize that the speaker (who said this word that is written on shown) is not using his/her brain right now, which can indicate cuteness or a state of being stunned by something extremely cute. Or it is simply used to make an emphasis, making people notice it more. Likewise, this word : フレンチガール uses katakana to emphasize (some stereotypical feature in this case). This word was found on a women's fashion magazine cover. I think in this case, in katakana form, the word kinda implies that this ガール is pretty, cute, stylish, thin ..etc. If instead it was written in the normal way (in combination of katakana, hiragana and kanji), it would still mean a young female from france, but it would sound plain and sluggish.
I think the textbooks generally explained katakana in similar but not very detailed manners. I also heard that katakana is the least commonly used form of syllables in the language (rumor?). So I guess some people may think it has an outdated or less important place in the language. But it looks like that this is changing. The usage of katakana has been evolving with time. Katakana usage can sort of reflect how Japanese culture regarded new concepts and renewed old images. Maybe whenever a usage of katakana can serve a desired purpose,it will be kept, otherwise it will be abandoned. Following this logic, we are in an experimental stage now. Some day in the future when we look back, katakana may even become a logo of this era, a time when people prefer to use katakana to express everything. But we never know how far this katakana trend will go before people find it boring again and start exploring new forms. It amazing to me that the Japanese language can be this flexible and renewable. And I guess, therefore, it is a difficult topic to include in any textbook, because there is no definite way of talking about when and how to use katakana in Japan right now.
