Why did the author choose a particular name for the visitor? (Framton Nuttel)
We can assume that Saki (H.H. Munro) chose a particular name for the visitor in "The Open Window" for the same reasons that most authors give names to their characters: it adds realism (most real people have names) and it makes it easier to identify the characters. Imagine how confusing it would be if you read a story about Man #1, Man #2, Woman #1, Little Boy #1, etc. In fact, one experiences such confusion when reading Paddy Chayefsky's play "12 Angry Men," in which the twelve members of a jury are referred to only by numbers.
Why did Saki choose the particular name "Framton Nuttel"? Now that's an interesting question!
I was unable to find any information about the name Framton. The name Nuttel, however, sounds like the word nuts or nutty, which means crazy or mentally unstable. Perhaps the author gave us a hint about Mr. Nuttel's unstable personality in his name.