Why do you have Kanji? You can write everything, using Hiragana and Katakana, can’t you? Kanji is way too hard to learn. Japanese people should not have Kanji.
There are a couple of reasons why Kanji is used.
The biggest reason is that it is much easier to read and understand a sentence with Kanji (if you know them) because each Kanji symbol has its own meaning.
Both mean ‘I studied Japanese at school on Monday’.
You can recognise and understand each word very quickly in the first sentence thanks to Kanji.
On the other hand, it is very difficult to do the same thing in the second sentence as it is written all in Hiragana.