Ｈiragana is the natural Japanese alphabet and any non-foreign word can be written in hiragana. Ｋatakana is used to write foreign words, such as non-japanese-korean-chinese names, name of cities and other places, brands and whatnot. Ｋanji are Chinese characters which are used to give meaning to the words, whereas us learner of Japanese would prefer not to deal with it and stick with hira+kata only, it is indeed essential to learn Japanese, because there are world that are written in the exact same way in hiragana but they are written with different kanji(s) and therefore they have different meaning, I just can't think of an example now... as I'm still very new to the language. Also Kanji is meant to be a helper while reading, after people get used to it you can read through words, whereas if it was written in pure hiragana it would be harder to know where a word ends and where the other starts, Kanji make texts compact, for example samurai さむらい word would written in just 1 Kanji 侍