For most Portuguese speakers there is a certain diffuculty in understanding the difference between these two prepositions. Native English speakers may ask 'why' when to us it is perfectly normal, but to a Portuguese speaker who uses the preposition 'para' which can mean both it can be confusing.
Although Portuguese does distinguish between to and for it is not normally uses in every day language example: Pedro dá este livro para Maria (Pedro give this book to Maria) can also mean (Pedro give this book for Maria). One can remove the ambiguity by saying Pedro dá este livro à Maria (Pedro give this book to Maria - the accent over the 'a' means the joing of the preposition 'a' or to and the definite article 'a' for a feminine noun. Sometimes removing ambiguity can be over com when reverting back to the language of the student to understand his/her confusion.