[retrac]

The underlying distinction is about mass versus countable nouns.

You can't refer to a sheet of paper as "a paper" (that refers to something else - a completed document). And you can't call a piece of furniture "a furniture", nor can you ask for "a scissors". They are mass nouns, inherently plural, referring to an abstract, indefinite amount of something. (Or in English, something coming in pairs, like scissors.) To refer to a specific instance, you need to use a determiner like "a piece of paper".

Or a head of cattle. You can't have two cattles. You have two head of cattle. Cattle come in a herd with an ambiguous number of them Other nouns are count nouns, and can be directly counted; they're definitive. A cat, two cats, four cats. Count nouns go with "fewer". Mass nouns go with "less". Over-educated writers might make a distinction here where "fewer cattle" is visualizing a few individuals, while "less cattle" is visualizing a smaller herd. I do think that's overly pedantic. Very few people make that distinction cleanly.

Some languages have no concept of count nouns at all, all nouns are mass nouns. Some languages have no concept of mass nouns, and all nouns are count nouns. Or nearly so. English has and uses both. Some cases are sort of blurry or unclear. "Six rains" = it has rained six times. That's maybe grammatical, but it's very unnatural in English. We do not feel we can count the times it rains, that way. It has to qualify a word that can be counted, like "times" - a pattern so common we get abbreviated counter words like "once" and "twice". Most languages have a touch of both patterns. Even in Chinese, which supposedly has no count nouns, there a few places when you do just count things directly.

So, no, it's not necessary. But if your language does make the distinction, it's very common for agreement patterns to show up based around that distinction. Akin to how French adjectives agree with their noun in number and gender.

[thaumasiotes]

> Even in Chinese, which supposedly has no count nouns, there a few places when you do just count things directly.

Are those things nouns? Days, for example, must be counted directly, except that the grammar is very clear that 天 is not a noun at all.

月 can be used with or without a measure word, but the obvious analysis would be that it is a noun if used with a measure word and not if counted directly, much like 人.

(Preemptive counterargument: is "right" a noun or an adjective?)

[bobbylarrybobby]

This did not address the question of why distinguish between mass and count in “less/fewer” but not “more”.

[Skeime]

One nitpick: Grammatically, mass nouns are not (necessarily) plural. You’d say “Paper is made from wood.”, not “Paper are made from wood.” (Of course, the pair words like scissors are different, i.e. “My scissors are in my backpack.” (I also seem to recall that there are non-pair mass nouns that only exist in plural form, but I cannot think of any examples.)

[tuatoru]

That's not a nitpick; that's agreement.

Less water, fewer onions.

Edit: not every word ending in "s" is plural. Some of them were just stolen from other languages and ended up with a final "s".

[Skeime]

You said in your post that mass nouns are “inherently plural”, which grammatically they are not. I suppose you were referring to the meanings of those words, though.

[Stratoscope]

> nor can you ask for "a scissors"

But you can ask for "the scissors".

Why does that sound OK but the other does not?

(Not debating, genuinely curious.)

[Skeime]

Because scissors is plural, which does not have an indefinite article. You can ask for scissors, like you can ask for documents. (And you cannot ask for a documents, only for a document.)

[tuatoru]

A pair of scissors, a pair of trousers. I shortened the trousers with scissors.