That's why, in part. It makes word boundaries easier to see, since Japanese doesn't use spaces. Kanji also help to differentiate between homophones, which look the same in kana since kana is phonetic. See 神(かみ), 髪(かみ), and 紙(かみ).
Because they're easier to read. Some words are spelled the same in writing, and since tone cannot be distinguished in writing, kanji is used. It makes it easier to tell the difference between words and some grammatical elements, so that it would be more widely understood.
Similar to the ははははながすき example, without kanji, I cannot read a thing and Japanese writing has no space unlike English where I am now typing and entering the space bar oritwilllooklikethiscrazymesswithoutspacepunctuationandcapitalletters.
(in case you couldn't read the mess: "or it will look like this crazy mess without space, punctuation and capital letters.")
Which is why Japanese uses kanji and we people writing stuff with the Latin alphabet use space in between words. (Or no one is able read and understand anything. The power of kanji, space and punctuation!)
Because past a certain point, it's impossible to remember words without kanji. Kanji are like some of "memory node"?
If you properly study and master a kanji, it can allow to you learn and remember more easily up to maybe a hundred words? And you don't even need to study and remember every words. For example:
Japanese don't remember every single words. But if you show them a word in context with kanji, they can guess its meaning. They can't recall every words but they are still able to understand the meaning of extremely uncommon words.
And also, it's just easier to learn words when you know enough kanji. Like it takes less memory and effort once you're past a certain stage. Especially, if you study the reading as well.
I don't think the lack of spaces is a satisfactory answer. They could start using them if they wanted to switch to hiragana. The homonym issue is a bit more solid, but even then, people can communicate verbally without issue.
The reason they still use kanji is that there is no reason to change. Literacy is not a problem in Japan. Everybody who has gone to school in Japan can handle it. There would be no advantage to switch, only disadvantages.
Sure it's a big hurdle to learn if you didn't grow up learning it, but it is not impossible. Plenty of people have done it.
The way we learn Japanese, hiragana and katakana first, then kanji later, is the opposite of how the writing system developed.
The rough, semi accurate history, goes kind of like this:
At first the Japanese borrowed the Chinese writing system and made it kanji. They mapped the words onto that. Later on they chose some of those kanji to be used for mapping sounds onto. Even later those got converted into a script like form that became hiragana. Katakana followed a similar path.
At first, people resisted using hiragana at all, and then it was largely only used by women. So woman wrote in hiragana, and men wrote in kanji. Over time both started to use the mixed system, which incorporated the strengths of both systems.
So kanji is used, because it is the original writing system. Where it made sense to use kana instead of kanji, those changes largely occurred. This is a constantly evolving process, which can be observed when words that were once written in kanji, start to become more commonly written in kana.
Interesting. Also, AFAIK some people believe an early phonetic writing system called Jindai-moji (神代文字) existed in Japan even before the introduction of kanji.
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