We use one (singular) and ones (plural) to avoid unnecessary repetition.
*See those two girls? Helen is the tall one and Jane is the short one.
*Which is your car, the red one or the blue one?
*My trousers are torn. I need some new ones.
*See those two girls. Helen is the one on the left.
*Let’s look at the photographs. The ones you took in Paris.
We often use them after Which ... in questions:
*You can borrow a book. Which one do you want?
*There are lots of books here. Which ones are yours?