Yes, why does Japanese need both “ichi, ni, san, shi …” and “hitotsu, futatsu, mittsu, yottsu …” for “one, two, three, four …” when practically every other language has just one set of names for the numbers?
It doesn’t NEED that, but it’s an artifact of the language. You may as well ask why English needs so many specific damned terms for groupings of things (a “pod”, a “pride”, a “pack”, a “band”, a “flock”, you get the idea). Japanese counters are basically just the answer to English’s grouping words. Hell, if I wanted to give an even more obvious counterexample, we have “one, two thee” and “mono, duo, trio” and “uni-, bi-, tri-”.
More specifically, “ichi, ni, san, shi” are Chinese-origin, “hito-, futa-, mi-, yon…” are Japanese-origin. I think which one you use depends more heavily on the origin of the counter word than anything (though I can’t prove that). In addition, with notable exceptions (i.e. yon is very common for “4”) the Chinese numbers are used much more often anyway.
ETA: Though I can’t really defend the fact that counting people goes “hitori, futari” and then arbitrarily switches to “sannin[…]”. Or why 20 years old is randomly “hatachi” (okay, that one is because it’s an important age in Japan, but still).
I’m curious on how that breaks down by part of speech, my guess is that adjectives are mostly Japanese, while nouns (and maybe verbs) are mainly Chinese, but that’s a barely-educated WAG.
Actually, the verbs are the bits that are almost certainly Japanese in origin. There’s an easy way to break down the origins, though I don’t know if it’s 100% accurate from an academic point of view. There are a minimum of two readings for the characters: onyomi and kunyomi. Onyomi are the loan-words from Chinese. Most of the compound words are made up of onyomi. Kunyomi are the native Japanese words that became associated with a borrowed kanji character that had the same or similar meaning. Verbs are overwhelmingly represented by kunyomi. The ones that aren’t are usually [abstract concept or noun]+する (suru, which is the rough equivalent of the English verb “do”).
Here be the stats (the stats come from a list of basic vocabulary for schoolchildren, so the overall results are somewhat different from my last post, which used magazine articles as a data set.):
Native Chinese Foreign Mixed Total
Nouns 6,004(29%) 12,504(59%) 1,415(7%) 1,070(5%) 20,993
Verbs 3,267(94%) 0 0 224(6%) 3,491
Adjectives 424(92%) 0 0 37(8%) 461
Adjectival nouns 146(32%) 266(58%) 23(5%) 24(5%) 459
Adverbs 1,034(89%) 80(7%) 0 41(4%) 1,155
Cunjonctions 64(98%) 1(2%) 0 0 65
Attributives 30(79%) 5(13%) 1(3%) 2(5%) 38
Interjections 109(89%) 0 9(7%) 5(4%) 123
Compounds 66(79%) 0 0 18(21%) 84
This actually, sort of, goes back to the original question in this thread. The reason why words of Chinese origin are so overwhelmingly nouns (or adjectival nouns like 奇麗 (kirei – beautiful/clean) or 丈夫 (joubu – solid) ) is that the distinction between noun, adjective and verb is not as clean cut in Chinese. The same word, like the OP’s “代表”, can function as a noun or a verb.
In Japanese, these loanwords all get classified as nouns, but with they can still function as verbs, adverbs or adjectives. For instance, you can have:
日本の代表を選ぶ -> To chose a representative of Japan.
日本を代表する -> To represent Japan.
日本の代表的作家 -> A representative artist of Japan.
代表的に活動する -> To act representatively.
In the list above, though, that all gets counted as a single noun.