Actually, you don't have to use Kanji. You can write Japanese only in Hiragana. Hiragana are phonograms, while Kanji are ideograms. Therefore, Japanese sentences with Kanjis are easier to understand. There are lots of words that have same sound. If you write Japanese with Kanji, you can avoid being confusing. We use Kanji a lot. Because we can make meanings of sentences more clear and the length shorter.

There's a good artilce on that. Japanese writing system https://en.wikipedia.org/wiki/Japanese_writing_system Il y a un bon article à ce sujet. Écritures du japonais https://fr.wikipedia.org/wiki/%C3%89critures_du_japonais