Spanish, Japanese, and German
I've reached a relatively high level in Spanish and mostly keep up with it by reading and doing a vocabulary deck. I spend the most time with Japanese when it comes to actually learning. My progress with German has been slow and shitty because I hate the textbook I'm using and can hardly bring myself to slog through it.
>Explanation as to why they're used today
As you probably know, Japanese doesn't use spaces. For most words, this isn't a problem; content words are (usually) written (at least partially) in Kanji, and grammatical morphemes are written in hiragana. This works well; native Japanese speakers prefer it over other kinds of writing because they say it helps them understand passages faster, and I have come to agree with that. But for loan words, this is more complicated. You can try to write them in Kanji based on their meaning, and Japanese people used to do this, but because Kanji have a relatively fixed set of ways they can be read outside of using them this way, they usually lead to confusion about pronunciation. You can try writing them in Kanji based on those readings, but this leads to confusion about meanings. Luckily, Japanese has two sets of phonetic characters: hiragana and katakana. Originally they were more or less equivalent to cursive versus print; i.e. they had no difference in meaning and were simply used by different social groups (women tended to be the primary users of hiragana), but over time they were relegated to different functions. Hiragana is used for grammatical morphemes and native or sino-chinese words if not otherwise written in Kanji. Katakana is mainly used for recent foreign words for which there is no Kanji, which combines the benefits of separating semantic and grammatical content that native words have with the benefits of not leading to confusion in pronunciation or meaning. Katakana is also often used to distinguish onomatopoeia-words, and can be used to replace Kanji or hiragana spellings in some contexts to convey emphasis or a casual tone.
Japanese people feel strongly that Kanji-kana mixed script is, overall, better for reading once you've learned it, even if it's harder to learn at first. The fact that it separates the content words from the grammar words very clearly without spacing them out is probably the reason for that, and as far as I know there is some evidence that it actually does make a difference. Also, Japanese is a language with a lot of (near) homophones. In speech they are (partially) disambiguated by pitch accent, but even kana doesn't represent represent that in writing, and even if they did, it wouldn't solve the problem completely and a lot of homonyms would be created.