Expression #1: Sound Effects from もののけ姫 Film Comic pp. 34, 64-65
In one of the fight scenes of the film comic for Princess Mononoke, various sound effects like "ガーン," "ゴバア," and " ドオ." They are written in bright red and violently scratched onto the page. I think the reason the author chose to use katakana was to make the sounds more dramatic. Katakana appears more angular and edgier, making the sound effects look kind of scary and cool at the same time. They're also very simple, which makes them good for expressing emphatic sound effects. Some sound effects in the comic, like "ざゆ," are written in hiragana. This is supposed to be something slithering across the ground, so I think because it is mean to recall a softer sound, the author chose to express it in the more rounded hiragana (also in blue). Though these katakana words are sound effects, I would categorize them in the "onomatopoeia" group, because they are sounds that represent actions, just like onomatopoeia.
Expression #2: リンゴとハチツとる〜りとけでる; Food Box
This is a picture of a box of caramel apples I found online-- it is an example of what I think is one of the most interesting uses of katakana. Although both "りんご" and "はちみつ" are both normally written in hiragana or kanji, but in this case they are in katakana. I believe this is an example of what some of the textbooks refer to as "emphasis." Having the main nouns of the package be in katakana probably makes them easier to read-- now instead of a long string of unbroken hiragana, there are two katakana words separated by a hiragana と. This makes the words distinct.
Why do the textbooks all have different explanations?
Although the textbooks were mostly similar in explaining katakana and when to use it, there were some minor variations. For example, only some of the textbooks mentioned "emphasis" as a reason why katakana would be used, although I've found that this is quite a common reason. I think there is so much variation because the rules about when to use katakana are not very hard or fast. Katkana is useful for many things, and can be employed at a writer's discretion. It is hard to categorize the various ways katakana is used simply because there are so many. As a result, I think most textbooks take the extremely common cases of katakana usage and present them as being essentially the only ones, because this makes it easier for first-year students to comprehend. Some textbooks go more in depth with this than others. One author might decide to mention the top four reasons for using katakana, while another might decide to only list the times when katakana is used almost 100% of the time, such as with loan words or onomatopoeia. There is a lot of mystery that surrounds the proper usage of katakana, and I think each textbook does the best it can to make the decision seem simple for beginning Japanese learners.
Sources: Nakama, Wikipedia, print-outs of other textbooks, http://www.sljfaq.org/afaq/katakana-uses.html, すずきさん
