Why use 'a' instead of 'para' or 'por'?

Question

In the examples of  "¿Quién prepara la fiesta a Sofía?" and "Nosotros les preparamos la cena a nuestros invitados."
Why use 'a' instead of 'para' (for) or 'por' (on behalf of)?
Ricky

Answer

Hola Ricky
Sometimes "a" has the same function as "para" (for), which is the case in this sentence:
"¿Quién prepara la fiesta a Sofía?" You could also say ""¿Quién prepara la fiesta para Sofía?"
In the second sentence you mention, "Nosotros les preparamos la cena a nuestros invitados.", the indirect object "nuestros invitados" is referred to twice: with "les" (to them) and with "a" (to our guests). Sometimes we admite the double indirect object in a sentence. Here are other similar sentences:
"Le he regalado una flor a Carmen" (I bought a flower for Carmen.)
"Les compré a los niños unos abrigos nuevos." (I bought the children some new coats.)
I hope this clarifies your doubt.
Inma