What is the purpose of the word "les" in this sentence?

What is the purpose of the word "les" in this sentence?

What is the purpose of the word "les" in the sentence "Él les está enseñando inglés a las niñas"?

It seems that "les" means "them", so I would expect that the sentence should either be "Él les está enseñando inglés" or "Él está enseñando inglés a las niñas". I don't understand why "les" is there if "las niñas" - is already there.

The verb enseñar can take both a direct object and an indirect object.
What is being taught? Inglés. To whom? A las niñas.

It is possible to leave out the indirect object pronoun (les) when the indirect object (a las niñas) is supplied, as you expected.

Using only the indirect object pronoun is also possible, but only when there's no ambiguity. The third-person indirect object pronouns (le and les) can reference either gender (masculine or feminine) and any third person. So, when composing such a sentence, the speaker needs to determine if the third person (or persons) being referenced will be understood by the party being addressed.

In this case, the speaker chose to use both the indirect object pronoun and the indirect object.

Whether this was because the speaker figured there might be some confusion or was simply adding emphasis (or was just being redundant, which is perfectly fine in some circles), we don't know. Only the addressed can tell us.

This redundant indirect object pronoun is the norm in Mexican Spanish (and probably in many Latin American countries). We don't feel a sentence is complete without it. :D

I can vouch for four Central American countries. They love being redundant, too.