I picked the following three phrases in Katakana for this analysis project:
（１）ナイーブ (loanword, naive)
source: http://www.naive-home.jp/
Interestingly, I found this Japanese body works brand called ナイーブ. In English, the direct translation of ナイーブ is "naive", but it seems the meaning of the Katakana word has changed a little bit over the years to also imply "tender", "refreshing", and "young".
（２）スナック (loanword, snack)
source: http://blog.goo.ne.jp/hide-kurach/e/5c71c84772787d8d28c35d8692a21f42
The English word that corresponds to スナック is obviously "snack", but the meaning of スナック has expanded to include "bars" where people go have a drink and hang out after a day's work. The picture in the URL above is perhaps that of some popular bars in Japan.
（３）ピカチュウ：(Onomatopoeia, emphasis, Pikachu)
source: http://bulbapedia.bulbagarden.net/wiki/List_of_Japanese_Pok%C3%A9mon_names
I used to play a lot of video games, and one of my favorite is Pokemon. ピカチュウ is perhaps the most well-known Pokemon throught the world. Like other Pokemon, its name is also written in Katakana and resembles the pronounciation in English.
For another reason, the character of Pikachu is sort of foreign to Japanese people (it's not a human character in real life anyways), and usually the names of foreigners are written in Katakana to emphasize their foreign origin. Therefore, the usage of ピカチュウ has dual purposes: Onamatopoeia and emphasis.
(4)スズキ (emphasis (unconventional usage), Suzuki)
Suzuki is a Japanese car maker, but why is it that its name is written in Katakana on its website? Well, Katakana names usually sound stylish, international and of foreign origin to Japanese people. As an influential international player, Suzuki may want to distinguish itself among other Japanese car makers and promote itself as stylish and cool. Also, the name will naturally stand out in newspapers when put together with hiragana phrases that surround it.
Textbook usage analysis:
Sometimes Katakana is used for purpose of *emphasis*, but this usage is sometimes less focused on in Japanese textbooks, like the one Matsui sensei showed us. In contrast, usages for loanword and onomatopoeia are well explained.
