hiragana is for japanese words and names: katakana is for foreign words and names: for example miller is american name we write it in katakana adnan is pakistani name we write it in katakana. hiroshima(cityname),mayomi(girl name) written in hiragana.

Like other people explained, katakanas are basically used for loan words. Also we use katakanas for animals and plants, such as キリン（麒麟）,バラ（薔薇）etc. Names for animals and plants often consist of difficult kanjis. So, in order to unify orthography of these names, it was decided to use katakanas officially. Or when people are not sure about difficult kanjis, sometimes they write katakanas, not hiraganas, because hiraganas might be buried to a sentence. I guess this happens not only children but adults. The above reasons explain well why we use katakanas. But, there are some other reasons and I'm going to explain with examples: ・ボケとツッコミ(funny man and straight man in a comedian duo) using katakanas for hiragana words not to be buried. ・東京チカラめし(a restaurant's name) using katakanas to make it catchy ・スタッフ (instead of 店員（てんいん）or従業員（じゅうぎょういん）) シェフ（instead of 料理人) using katakanas to sound more stylish or attractive I think that's about it. I hope this helps you.

Also, sometimes in games meant for small children, some expressions are written in katakana even though it is not a loan word. The reason why they do this is because at that age, they might not have mastered the kanji yet. So whenever you see a katakana and it is not a loan word, this could be another reason :)