We've covered that the Japanese writing system uses a combo of hiragana, katakana, and kanji.

Is it really necessary to use three types of characters?

Kind of. Consider the following Japanese sentence:

日本語が話せるフランス人は多いです。
にほんご　が　はなせる　フランスじん　は　おおい　です。
There are many French people who can speak Japanese.
Literally: "Japanese (language) + が + can speak + French person + は + many + です."


This sentence contains hiragana, katakana, and kanji:

日本語が話せるフランス人は多いです。
にほんご　が　はなせる　フランスじん　は　おおい　です。
There are many French people who can speak Japanese.
Literally: "Japanese (language) + が + can speak + French person + は + many + です."


One argument for including all of these different types of characters is that it helps us to distinguish words from one another. After all, Japanese doesn't have spaces:

日本語　が　話せる　フランス人　は　多い　です。
にほんご　が　はなせる　フランスじん　は　おおい　です。
There are many French people who can speak Japanese.
Literally: "Japanese (language) + が + can speak + French person + は + many + です."


↑ See how colors change between all words, with the exception of です at the end?

One of the uses of hiragana is that it is the character type of choice when performing grammatical functions. That's why our particles (が and は [pronounced wa, not ha, when a particle]) are in hiragana. Our verb ending in 話せる (はなせる // can speak) is also in hiragana, as is the ending of our i-adjective 多い (おおい // many), and the copula です (see Bunkai Beast for more about copulas).

Our katakana characters are used for the foreign loan word フランス, "France," which comes to mean "French person" when we attach the kanji meaning "person," 人. So, we have フランス人 (フランスじん // French person).

Then all of the kanji in the sentence express not only sounds, but also meanings:

日 = sun/day
本 = origin
語 = language
話 = talk
人 = person
多 = many

(Note: See how Japan, the "sun-origin" country, is the Land of the Rising Sun [日本 // にほん]? Cool, huh?)

Once you get used to reading Japanese, having kanji makes it much less difficult to decipher sentences. For example, if a Japanese teacher wrote the above sentence using only hiragana and katakana (which, sadly, many of them would), it would look like this:

にほんごがはなせるフランスじんはおおいです。

I think it's safe to say that I'm very good at Japanese. But this (↑) makes my head hurt. This (↓) does not.

日本語が話せるフランス人は多いです。

However, just saying that kanji helps us to set words apart when reading is not a very good excuse, is it? Why can't they just use hiragana only with spaces?

にほんご　が　はなせる　ふらんすじん　は　おおい　です。

I'll be honest. I've yet to hear a fully convincing argument as to why this would not work. After all, Korea did something very similar when they stopped using Chinese characters in their writing several hundred years ago.

Let's take it on step further. Why not just write in romaji (i.e. using the roman alphabet)? That way, we wouldn't even have things like that irregular reading of "wa" instead of "ha" for は：

Nihongo ga hanaseru furansujin wa ooi desu.

Here's the weak defense against opting for spaces instead of kanji: There would be too many homonyms.

To give an example, はな can mean either "nose" or "flower" in Japanese. Kanji helps us to clarify that they are different:

鼻（はな // nose）
花（はな // flower）

If we used only hiragana in our Japanese sentences, then we might have trouble translating this:

はな　の　え　を　かいた。
I drew a picture of a nose/flower.
Literally: "nose/flower + の + picture + を + drew."


With kanji, it is immediately clear which one we are talking about:

花の絵を描いた。
はな　の　え　を　かいた。
I drew a picture of a flower.
Literally: "flower + の + picture + を + drew."


鼻の絵を描いた。
はな　の　え　を　かいた。
I drew a picture of a nose.
Literally: "nose + の + picture + を + drew."


We can even say that kanji helps us to differentiate that we're talking about 描いた (かいた // drew) and not 書いた (かいた // wrote).

I think this is a somewhat weak argument. After all, people understand when you're saying sentences like this in person even though they can't see what kanji would be used. People are very good at inferring context. There would be clues in a given situation that make it easier to figure out which words are being used. The most obvious clue in the sentence above, for example, if that 絵 (え) means "picture" or "painting." So, obviously we're talking about "drawing" or "painting" and not "writing."

I would argue that the following is the true reason Japanese people insist on sticking to their current writing system: They like kanji.

In other words, kanji continue to be used for emotional, not rational reasons.

When someone forces you to study something your whole life, it feels pretty cool when you know it but the rest of the world doesn't. It feels nice when people complain that something is too difficult or complicated but you understand it.

Also, kanji can be quite beautiful. And writing them is an art form.

Complain if you want, but this will be much more enjoyable if we start to appreciate the beauty of kanji, too, and to pursue that awesome feeling of having worked hard to understand something.