September 7, 2002:
Why are Merry and Pippin there?
It would be tough to answer this question from the movie alone. Comic relief, basically? To contrast their good-nature with Frodo's seriousness? It's as if they got drunk and woke up in Sam's backpack. You have the sense they don't understand where they are or why.
The books are very different. The Hobbits know all about the Ring, the danger, and what has to be done. Yet they refuse to allow Frodo to face those dangers alone. At their deepest thematic level the books are about loyalty: friends who follow each other into Hell for friendships' sake. Maybe this has something to do with their lasting emotional power.
Loyalty between friends is also the thematic basis of Star Trek, Harry Potter, and The X-Files. Substitute families. It's possible that, like The Lord of the Rings, this has something to do with these fictional worlds' ability to earn their audiences. People turn to them looking for something lacking in real life.