for us, Kanji is more useful in writing so that they can understand what you really mean bc each Kanji has a meaning. and also helps to make spaces esp like on essay or something like that.
and katakana, it's used for things come from other country such as "computer = コンピューター", "Hamburger = ハンバーガー", "Hanger = ハンガー", and so on.
and Hiragana, it's basic characters. You MUST need to remember the whole characters bc they are everywhere like on news papers, subtitles in movies, and so on.
ask me anything if you don't get what I'm saying^^