I have been studying Japanese for a little more than a year now,and I've been learning kanji for a few months.
I understand the basics about readings, but I don't really get the historic reason of why the Chinese readings were imported along with kanji.
If I understand correctly, back then Old Japanese was already a language by itself, only lacking a writing system which was imported from China.
Now, I understand that maybe they took some kanji which represented things for which they didn't have a Japanese word, and thus used the Chinese sounds they heard.
But seeing the huge amount of compound words which use onyomi, it's impossible to think that none of them were used before with their native pronunciation, especially since there are also a lot of words which having more that one kanji use kunyomi. For example, some vocabulary words I've learnt recently are: 大雨 ( おおあめ, kunyomi) and 小川 ( おがわ, kunyomi).But 火山 is かざん (onyomi).Maybe the concept "volcano" didn't have a word for itself in Japanese, but why create it imitating Chinese instead of joining their native words (ひ and やま)?