I think it was told by the junior hobbits because they had the direct connections to Aragorn, Gimli, and Legolas.
It is very funny and rather dramatic not to know what happens... and then have Merry and Pippen at the door (smoking and sleeping)... It strengthens the love between the character and elevates the characters above the action.
Maybe Tolkien wasn't satisfied with his writing of the "action"... couldn't do it justice, etc., so reverted to narration. (That's what I'd do, but I'm no genius and he writes everything so well).
I think that this might be it. As I remember, one of the ways they increased suspense in novels written as an exchange of letters, as Pride and Prejudice was in part, was to have characters anxiously awaiting news, then have a character or letter arrive, telling the story.