"I send letters to my English friends."
Translation:Yo les mando cartas a mis amigos ingleses.
Because what you're really saying here is:
I send letters 'to them'.
to them = les.
les = a mis amigos ingleses (to my English friends) which is clarifying what les (to them) actually means.
Unlike in English, in Spanish you always use the indirect object pronoun, even if you are explicitly stating the indirect object of the sentence. If you already include the indirect object explicitly (like "English friends" here) then the pronoun is just a placeholder but the grammar says it has to be there.
https://www.spanishdict.com/guide/indirect-object-pronouns On my own search I found this article, which provides a lot of helpful explanation and a quiz for practice. Unfortunately I still can't answer the heart of the question here, which is why "les" is required before the verb if you are going to say "a ellos" afterwards. I will ask my friends down here in Peru and report back.
If the object you're talking about is just a personal pronoun (like "a mí", "a ti", "a él", and so on) you also need an object pronoun before the verb. If the object is an actual noun, you don't need it.
- Veo a mi hermana. (noun object)
- La veo a ella. = La veo. (pronoun object)
- (Le) escribo una carta a mi hermana. (noun object, le is optional, but often used)
- Le escribo una carta a ella. = Le escribo una carta. (pronoun object)
Whether you use the object pronoun or not wouldn't change the meaning of the sentence, but the rules of the language dictate when they are used. Meaning, you could say "Veo a ella" without using the intial la and it'd still be understood to mean "I see her", but it's grammatically wrong.