ラーメン - ramen (from a Japanese food package)
トラ - tiger (from a website on Japanese animal names)
ライオン - lion (from a Japanese advertisement for the movie "The Lion King")
イマ (from a Japanese street advertisement)
These words written in Katakana particularly stood out to me because they seem to serve some other purpose besides just borrowing words from English. The word for "ramen" was borrowed from the Chinese word for ramen, but it is unusual to me especially because it is written in katakana so frequently when there is a kanji character for it. This word was probably borrowed and written in Katakana because ramen is has become so popular in places outside of Japan and China that "ramen" has become very common in the English language. Writing "ramen" in Katakana was probably popularized as ramen has lost its uniqueness to China. In this way, ラメン shows how Katakana can be used to write Chinese words that aren't really considered to be exclusively Chinese words anymore.
I found it unusual that the word for "tiger" was written as トラ instead of in Kanji or Hiragana even though it is a Japanese word. It was probably written in Katakana for the sake of consistency as most other animal and plant names are written in Katakana because they are more often borrowed from English. This brings me to the word for lion, which was written in Katakana, but was borrowed from English instead of just writing Japanese in Katakana. This is in keeping with the common usage of Katakana for biological/scientific names as words such as "lion" did not originate in Japan, given that they are not indigenous to the country. Thus it makes sense to use Katakana for these types of words. Although, Katakana is used for animal names, Kanji is also used. This is probably because some animals such as the bird (鳥) are more common in Japanese culture.
I found that the word for "now" was written in Katakana on a Japanese street sign. This is probably for the sake of emphasis in order to draw attention to the sign or advertisement. It makes sense to use Katakana because it has the effect of standing out from Hiragana or Kanji due to its more simplistic writing form. It also seems to be more informal than the more traditional writing systems. For this reason, I can see why the Japanese word for "now" was written in Katakana instead of Hiragana especially when the sign reading "イマ！" is in the midst of many other signs written in Japanese. Furthermore, Katakana is more flexible than Hiragana or Kanji because it covers a larger range of sounds such as "fa, fi, fe, fo" or "va, vi, vu, ve, vo." Because it is a phonetic writing system it is able to accomodate for these sounds which are not common to the original Japanese language. Thus, it is very convenient to use for other purposes such as loanwords, or emphasis in order to mark words that are not common to Japanese.
Each textbook is different in explaining Katakana because there are many uses to the writing system beyond what most people commonly use it for. I think that sometimes Katakana is used more liberally than Hiragana and Kanji, thus allowing us to interpret the reason behind the usage (such as the words I just mentioned above). Katakana seems to be more flexible in its purposes, so it is unlikely that each textbook will come up with a unanimous way of approaching the writing system. Also, the textbook's explanation of Katakana also seems to depend on when it was written. For example, the textbook our class uses spells many vocabulary words such as "テーブル” instead of the Japanese word for table (表) which other textbooks may use. This shows how some English words are being more popularized, especially in the modern age with better global communication. Just as later editions of dictionaries tend to include more modern/slang phrases in formal definitions, I find that more recent textbooks are adding more usages of Katakana as the language evolves and new usages become more popularized. There are many different uses of Katakana, so as someone new to Japanese, I would familiarize myself with the various uses so that I know the appropriate situations in which Katakana is generally used.
