•Why it earns a slot
Why it earns a slot: it turns a bank note that is never once spent into the entire engine of the plot, and Twain uses the joke to expose how fast a city extends credit to a man it merely believes is rich.
A broke American is handed an uncashable million-pound note as a rich man's bet and discovers that just carrying it, never spending it, is enough to buy him an entire life.
This distillation is written from the freely available original, which is always the better read when you have the time: Project Gutenberg.