I’m still reading Treasure Island. The crew of them are about to set sail from England. As I read, the Letter from the Squire stuck out. Dropping a letter into a story essential stops everything (unless letters are the story structure). This letter was from the Squire who went ahead to Bristol to find and crew a boat that could make the trip to Treasure Island. Jim Hawkins stayed behind, and the Doctor went to London to find someone to take his medical practice while he was gone. The letter has alot of ground to cover because it essential fixes the problem with third person limited.
Third person limited is a point of view of a single (and therefore limited) person for the entire story. Jim Hawkins is our narrator but is not in Bristol or London so the letter catches the reader up on what is going on in Bristol. The letter explains that the boat and crew have been found and that, how fortunate, a one-legged man will be the cook. The one-legged man was useful in securing almost an entire crew. Since Billy Bones told Jim to look out for the one-legged man, the reader is also looking for the one-legged man.
Was the information in the letter necessary?
Without the letter, the author would have to create the same type of suspense (knowing something bad is there and expecting some hell to break loose because of it) as soon as Jim Hawkins got to Bristol. Without the letter, the reader is not dreading Bristol but looking forward to it.
How else could the author have provided that suspense?
The story takes place when written or oral communications were the common forms of communication. So either the letter or someone had to come back from Bristol with the news of the new cook and crew. If a person had returned with the news, the information wouldn’t be dropped in stopping action but the scene could include the action of the person talking with Jim or at the Pub.
If the author planned to drop more information in via letters and notes, then the letter sets the reader up to expect a new piece of information when one is found.