"katakana" is used in many occasions,primarily it is often used for imported words but is also sometimes used when you want to emphasize the word or make the word more comprehensible

That's a very keen question! Even though the word is not originated from foreign word, we sometimes write names of daily necessities, vegetables, fruits, plants and animals in katakana. Especially if kanji is relatively difficult. I agree Rintaro-san. In my opinion, when we read Japanese, we unconsciously recognize hiragana part(such as particles, verb endings etc.) as less important because it carries only grammar, not actual meaning; and kanji part as more important because it has its own meaning. If "kagi" was written in hiragana, it's hard to recognize the word in one look because it's "fused" with other hiragana parts. However, it takes time to write kanji, and sometimes the kanji might be unfamiliar. That's why katakana is used instead of kanji, I think.

文字をはっきりわかりやすくするためです。 ひらがなですと・・・ 「あ」と「お」、「さ」と「き」、「はと「ほ」、「わ」と「ね」、「め」と「ぬ」が似てませんか？ カタカナですと、文字の違いがはっきりしています。