Great question!
Quote:
Le estoy escribiendo un mensaje electrónico a mi madre.
Él le da flores a su mujer.
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I'm writing her an email - Le estoy escribiendo un correo
He's giving her flowers - Le da flores.
In the above examples,
le serves as the indirect object pronoun, showing who you're giving flowers to and who you're writing to, just like
her in the English version.
When you want to give more specific info about the person receiving the flowers or email, in English you remove the
her and replace it with the prepositional phrase "
to my mother".
I'm writing her an email to my mom.
He's giving her flowers to his wife.
In Spanish, when you add the prepositional phrase, you don't remove the indirect object pronoun. They are both needed.
Le estoy escribiendo un correo a mi madre.
Le está dando flores a su esposa.
It would be something like this literally translated to English:
* I'm writing her an email to my mother. 
* He's giving her flowers to his wife.
Both the
her and
to my mother are needed in Spanish; In English you must use one or the other.