The first sample is from a manga magazine called "Jump", which is well advertized in the anime Gintama. In the sample the word "manga", which is usually "漫画" in Kanji, is instead written in Katakana. Tatsumi Sensei mentioned that many young Japanese people use borrowed words in daily life even though some words exists in traditional Japanese because they think it is fashionable and fun. Therefore, it is very reasonable to think that they use Katakana here to represent the English word "manga" (which probably comes from the Japanese word) in order to give the advertisement a modern and fashionable feeling.
The second sample is the cover of a book that talks about the surprising difference between Japanese people in the western part and those in the eastern part. On the top, the word "びっくり", meaning "surprise", is also written in Katakana. From its pronunciation we can easily tell that this is not an English word. In fact, it is a Japanese word and is written as "吃驚" in Kanji.
In the corner of the advertisement, the word "こちら", meaning "here", is also written in Katakana. The reason of using Katakana here becomes obvious when we notice the exclamation mark right after the word. Katakana here might play a role similar to the all-capital-letter word "SURPRISE!" Writing this ビックリ in Katakana can to some extent reinforce the exclamative tone. Furthermore, writing the word in Katakana makes it stand out among other words in Kanji and hiragana and really gives the reader some surprise. A similar example of this is in the corner of the first sample. "コチラ" is like "HERE!" in English, making itself more eye-catchy.
The third sample is on the label of a facial cleanser. On the top of this label, there is "ニキビ", written as "面皰" in Kanji and meaning "pimple" or "acne". Instead of making the word more fashionable or more eye-catchy, the usage of Katakana here seems to make the word more subtle. Acne and pimple are often considered ugly and dirty. Kanji often has a very strong visualizing effect, since each Kanji is closely linked to a specific object, which can be easily visualized when the Kanji is processed by the brain. Therefore, instead of writing the word in Kanji, which gives people a straightforward feeling of "facial defect", writing it in Katakana make the reader feel like the producers are whispering the word to the customers and trying not to embarrass them. In this way, using Katakana here makes the label more subtle and polite. Another reason for using Katakana rather than Kanji here is that Kanji is too formally scientific for a label of a product. Googling "ニキビ" and "面皰", I found that "ニキビ" is more often used in daily life on like tips of cleaning acnes or advertisements for such products, while "面皰" appears more in professional articles, including wikipedia and articles discussing medical treatments on acnes. Our TA Aoki-san says she has seldom seen "ニキビ" written in Kanji. Probably Katakana is just the common and natural way to write it.
In the three examples above, we see different functions Katakana has in changing the tones under different circumstances: to make the word look fashionable and modern, to express exclamation, and to make words more subtle, polite, and easy for everyday use.
Aside from representing loanwords and sounds, the handout sensei gave us only mentioned the function of Katakana to represent the words the writer wishes to emphasize, examples of which are the “コチラ” in the first example and the “ビックリ” in the second. The textbooks did not mention that Katakana is also used to express special feelings or manners of speaking or just to look fashionable. I believe the reason for the textbooks not doing this is that the functions of Katakana in the samples above might be very arbitrary and specific, and their usage varies among different writers. Therefore, it is harder generalize these uses into one or two rules. There should be a lot more ways of using Katakana, but we could probably only learn them by really speaking and using Japanese in our daily life.
