"There are strawberries. Do you want some?"
Translation:Il y a des fraises. En veux-tu quelques-unes ?
You need quelques-unes to indicate that it is "a few" or "some" and it must agree with the object in gender and number. The en replaces "des fraises" or "strawberries".
Another way to phrase it is : Est-ce que tu en veux quelques-unes ? — Do you want some of them?
« Avez-vous acheté ces deux livres ? » « Non, j'en ai acheté un. » — Did you buy those two books? No, I bought one (of them).
The second sentence would be considered incomplete in French without the pronoun "en." In English you can end the sentence with "some" and it is understood that you mean (of the strawberries), in good French you must add the pronoun "en" to take the place of "des fraises" or "of the strawberries."