The main feature to notice about the table is that the punctuation marks are shown in relation to one another, in hierarchy. As you move up the scale you create more separation between statements, and more emphasis on the word or words just before the punctuation mark. Or needs to. Twain might have punctuated it several other ways. Man is the only animal that blushes or needs to.
Man is the only animal that blushes, or needs to.
All of these are possible, and each of them creates a subtly different effect. Notice how it seems to flow by all in one breath, so to speak, with no emphasis on the key word, blushes. It's pretty clear why Twain didn't choose zero punctuation. Yes, with good reason. Twain was writing literary humor, not a research report or a business letter, where readers might be confused or irritated by fragments.