When is explicitly knowledge of a kanji reading's category ('on', 'kun', etc) ever needed?

Question

When is explicit knowledge of a kanji reading's category (on, kun, etc) ever needed? By this I mean knowledge of any of the following mappings:

• (kanji, reading)-pair to category: eg being able to say explicitly whether かい is an on- or a kun- (or other) reading of 貝
• (kanji, category)-pair to list of readings: eg being able to explicitly list all on- or kun-readings of 正 (perhaps restricted to the list of important ones)
  • special case: being able to list a kanji's most common on-reading

My understanding is that the main use of such explicit knowledge is to be able to pronounce previously unseen compound nouns in a way that is socially accepted and reasonably likely to be correct, and for this only the 'kanji's most common on-reading' special case is needed. But are there other uses? Note that for the purpose of this question, I consider things like "exams in Japanese language class in secondary school" or "level X of the Kanji Kentei (漢字検定)" to be valid use cases.

The following posts contain related content:

• What's with this "On reading"/"Kun reading" thing? Is it important to learn both as a beginner?
• Is the difference between On and Kun readings greater than just the pronunciation of the character?

Answer 1

If you were born in Japan and are learning Japanese as a native speaker, you don't need explicit knowledge of on/kun-readings at all to be able to speak or read Japanese fluently. Well, likewise, no explicit knowledge of grammar is required, either.

In the third grade of elementary school, every Japanese child learns about the existence of on/kun readings, so it can be assumed that most Japanese people have a basic understanding of what on/kun readings are, just as most English speakers know what nouns or adjectives are. However, such knowledge has little to do with the ability to speak Japanese fluently on a daily basis. Most Japanese people rely on intuition rather than explicit reasoning about on/kun readings when guessing the pronunciation of unfamiliar words, and I believe they can do this even if they hadn't learned about on/kun readings at all in elementary school. Of course, such knowledge is necessary for performing well on exams, especially entrance exams.

On the other hand, if you are over 13 and wish to master Japanese as a second language, it goes without saying that having explicit knowledge of the grammar and the rules of kanji readings is useful. (Of course, simply memorizing a list of kanji readings without understanding their meanings is clearly very inefficient, and I basically agree with the advice that you should learn kanji readings naturally as you learn new words.)