Basicamente, emprega-se para não repetir o objeto.
A: Can you lend me a pen?
B: Sorry, I don't have one. (... have a pen)
A: I wish I'd have a small house.
B: One with a garden? (A house...)
A: I'd like to try on those shoes.
B: Which ones? (Which shoes?)
A: The ones in the window. (The shoes...)
No entanto, podemos omitir one(s) após which, this, that, another, either, neither e superlativos.
Which (one) would you like? This (one) looks good.
Let's have another (one).
Either (one) will suit me.
Só não podemos omiti-lo se houver um adjetivo:
This blue one looks good. (Não This blue looks...)
Quase sempre omitimos ones após these e those:
Erwin doesn't think much of these. (Mais natural do que ... these ones)
Não empregamos one(s) após my, your etc., some, several, (a) few, both ou a number.
Take your pen and hand me mine. (Não ... hand me my one)
Did you spot any UFO today? Yes, I spotted some flying nearby. (Não ... spotted some ones...)
I'll take both. (Não ... both ones)
How many apples did von Rundstedt buy? He bought six. (Não ... six ones)
De resto, o uso, nós o aprenderemos com a prática e com o tempo.