Tolkien has a book of "Unfinished Tales." In one of them, after Sauron is killed, Gandalf the White is recounting to Frodo and the other hobbits how he originally enlisted Bilbo in the journey chronicled in "The Hobbit." My theory is that this is what Frodo's character is being included for.
I am excited to see The Hobbit, but they are changing the story unnecessarily! It is my understanding that they have cast all kinds of characters that are not in The Hobbit and that they have not even cast BILBO or secured the same actor for GOLLUM, the two main characters! This is nonsense!