Gooood question my Dearr katakana is used to indicate the pronunciation of the words in foreign languages, due to the number of combinations is very limited in japanese language symbols. If only one of them is used, many confusions are induced due to serious collisions of combinations.

Strange question. why don't you use Latin letters instead of Arabic abjad? Japanese use two kanas because there are cultural and practical reasons. Generally hiragana is used to write grammatical endings and particles. katakana is for words borrowed from English and other european languages.(and some other words.. like animal names) both sets of kana have the same sounds.

katakana used for foreign words that has no original word in japanese , hiragana is the opposite . thats what i got when i was trying to learn japanese then i gave up :D