Suppose you're up against a flop of 688 and you hold something like 89TQK. How much of a favorite is the made baby boat 68xxx?
The surprising answer: barely at all. You have 12 outs twice, with a 42 card stub. You'll make the bigger boat 1 - 30/42 * 29/41 = about 0.495 of the time.
Of course, Villain might be able to make a low (if you're playing O/8) or hold some of your overcards himself, so your equity is not really 49%. But against something like 668JJ you might even be the favorite. That extra card really makes a difference.