When do you use Kanji in Japanese?

I hear many learners of Japanese have the same question that why kanji is necessary in Japanese.
If we want genuine answers to it,we should consider the relationship between Japan and China in the ancient times.
In a word,however,that's because kanji is useful.
First,there are a lot of homonyms in Japanese but we can write them as different words by using kanji.
Second,we can write Japanese sentences more shortly by using kanji than we do by using only hiragana and katakana.
Third,we can read Japanese sentences more quickly when kanji is used than when it is not.
Beginners may not understand the importance of kanji,but it is indispensable for Japanese.
The fact that some Japanese people tried abolishing kanji but they couldn't proves the importance.

Of course you can use hiragana. But it's very difficult to read in the long run. Many words have the exact same pronunciation, so they are written the same way in hiragana. Because of that you might have to guess the word to understand the meaning behind it. Kanji are clearer and faster, also shorter.

when the word is spelled with Kanji

Kanji is necessary to distinguish among homonyms. For example, 公社, 講社, 工社, 公舎, 後者, 校舎, 降車, 高射, and 巧者 are all こうしゃ in Hiragana. There are so many homonyms in Japanese that without Kanji we can't recognize the correct meaning of a sentence.

And, learning Kanji is not so hard for us native Japanese. The literacy rate of Japan has been the top of the world for several hundred years.

There are also other reasons.
1) We feel mixed Kanji and Hiragana sentence is more beautiful than Hiragana only or Kanji only (as Chinese) sentence.
2) If we abolish Kanji, then we can't access historical writings. Such inconvenience is intorelable because we have so many writings using Kanji of more than 1,000 years.
3) We can recognize Chinese writings without learning Chinese language.
4) Kanji is a idiogram and we can get the meaning at one sight.

It's indeed possible to write an entire Japanese sentence in hiragana. For example, many children's books are written in hiragana only. But Japanese school children are expected to to learn 1,006 basic kanji characters, the "kyōiku kanji", before finishing the sixth grade. And by the end of the ninth grade, they are supposed to know enough kanji to be able to read Japanese newspapers and literature.