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

Hola Ricky! First of all sorry for our late reply. We need to use the preposition "a" because we have previously used "les" as an indirect object pronoun and this preposition "a" introduces an indirect object refering to people ("our guests") in this case. However, you can omit this pronoun and use the preposition "para" instead too, for example, you could say "nosotros preparamos la cena PARA ellos". Both options are valid with a slightly change of meaning, like in English. The first sentence "nosotros les preparamos la cena A nuestros invitados" means "we prepare dinner TO our guests" and the second one "nosotros preparamos la cena PARA nuestros invitados" means "we prepare dinner FOR our guests". I hope this helps you to understand better! Silvia.