2 カタカナ words that I would like to look at are スエット (sweat) and インターネット (internet). I believe that these words are written in Katakana instead of Kanji and Hiragana because they are words taken from the English language (loanwords).
"Sweat" in the English language has the connotation of exercising, doing a sport, and being athletic. Thus, using a loan word for "sweat" is fitting for "Pocari Sweat," because even though it is a Japanese drink, it is also a popular sports drink that is sold in the international market.
"Internet" is also an English word, and thus Katakana is used in this case because it is a loan word. Even though "internet" is in English, its meaning is commonly understood. As we can see in the picture, the sign has English subtitles for "Shibuya" and "Information." We can deduce from that the sign is meant to attract not just locals but foreigners as well who need the help of an information desk and internet to get around.
I think the textbooks of Minna no Nihongo, Yookoso, Naama, Genki, Japanese for Busy People, and the grammar textbook written by wako Tawa have slightly different definitions for the usages of Katakana, because the reason why Katakana is used can vary from situation to situation. As in the two examples I used (sweat and internet), we see that Katakana is used because they are both loan words. However, the connotations and purpose behind each word differ.