Japanese writing system of Hiragana and Katakana are syllable based. For each syllable, there is a corresponding character. Hiragana system is used for writing words that are indigenous to Japanese language or Nihingo. Examples are : oishi, ibe, tsugi, Nichiyoubi, etc.
The Katakana system is the one used for writing borrowed or foreign words. Examples: cassette (ka-se-to), Christmas (ku-ri-su-ma-su), tape (te-po). But Katakana can also be used for writing indigenous or native Japanese words.
The Kanji system is one that uses one character for a particular word. It is easier to write a word of at least 5 syllables with one character than 5.
Life is made more convenient this way. I just don't know what other reasons are. Let's wait for native speakers.
Just imagine a word like cyclopentanoperhydrophenanthrene written as one or two characters. The days of the week are the best examples. Nichiyoubi, Getsuyoubi, Kayoubi, etc.